Asking questions is the most fun part of learning YHWH's word - and the most frustrating when we can't find an answer.
You asked:
"If the Great Multitude are coming out of Great Tribulation and still coming to heaven… rather than the Great Multitude came out of Great Tribulation after the 6th seal was opened, why aren't they called the souls of the Great Multitude like the souls of the dead saints under the altar in Rev. 6:9 who were martyred before the 6th seal was opened?"
I will answer you but first I want everyone to understand a VERY important concept and demonstrate it by showing some scripture details that will interest EVERYONE since this digression is all about the rapture.
Over and over in all Christian based forums (weblogs, Sunday schools, Bible studies, etc.) I keep seeing people trying to make scripture fit around their ideas of how they think it should be or were taught instead of changing their views around what scripture clearly says, regardless of what dogma or denominational toes get stepped on.
We must always recognize the difference between our conclusions and what the word says. What it says must be kept separate from what conclusions we might come to or we will quickly get confused or start to doubt the scriptures when we find clashing concepts based our conclusions. Always be willing to doubt your conclusions. Changing our beliefs or mental constructs is hard on our emotions and pride but we must be ruthless in our search for the truth.
I will state for the record that many details of my Christian beliefs have changed over 37 years because I took the time find out for myself instead of believing tradition or what was being taught to me. I have found that a lot of what our churches are teaching is NOT what the word tells us.
Most of the time our English Bible translations are good enough to know what the truth is but sometimes they are not because many translators attempt to interpret at the same time instead letting the actual meaning come through even if it counters their beliefs or doesn't make sense. Therefore we must sometimes do some very detailed and difficult work in the Hebrew or Greek.
For instance, in Matthew 8:7 it is recorded:
And Jesus said to him, I will come and heal him. - AKJ
We don't know who 'him' is unless we read the two preceding verses and find out it was the centurion's servant. Past that, is there any doubt to what it means? No. In 17 Bible versions I looked at they are all nearly identical and match the Greek except the NIV which changed 'come' to 'go' but even that doesn't really change the meaning.
But here is a case where the translators have done us a disservice. Here are two versions from the Lord's Prayer in Matthew 6:13:
And lead us not into temptation, but deliver us from evil. - AKJ (from KJV)
And lead us not into temptation, but deliver us from the evil one. - NIV
In 18 translations I looked at, these two represent a nearly equal split between two ideas. The first conveys the idea of deliverance 'from evil' in general and the other 'from the evil one', whom we all assume is Satan - right? Actually the Greek here, and identically in Luke's account (11:4) as a second witness, tells us something a bit different - but a very important difference.
Both these translations have made assumptive errors. Leaving out the article 'the' is incorrect - it's there in the Greek as 'Tou' for a reason. Ignoring the word form of 'evil' (nominative) that perfectly matches why there is an article in front of it is dishonest. Adding the word 'one' is flat out wrong because IT IS NOT there! This is called interpretive translation and is only one step away from paraphrasing.
Only the Young's Literal Translation had the courage to relay it exactly from the Greek as "but deliver us from the evil" even though it doesn't initially make any sense until we find out what the Greek tells us what 'the evil' actually is.
The word for 'evil' here is Ponerou, from the base word Poneros (4190). It is in the nominative case which means it is a title. This is why many translators chose to add the word 'one' even though it is not there.
The definition of this word is 'full of labours, annoyances, hardships, toils, perils, a time full of peril'.
(www.studylight.org/desk/view.cgi?number=4190)
Putting it in context and adding some more definitions really turns on the light:
And lead [Eisphero = bring in, carry in] us not into temptation [Peirasmos, singular = proving trial], but deliver [Rhoumai = rescue] us from [Apo = separation, departing] {the} evil [Ponerou = time full of peril - "The Time of Evil"].
Then what does this mean? That's right! This is the Tribulation! He is telling us to pray that we escape 'from The Tribulation'! It could be accurately paraphrased as:
And do not bring us into the proving trial, but rescue and separate us from the Time of Evil.
The following verse in Luke is a companion verse to Matthew 6:13:
Watch [Agrupneo = to keep awake, watch, attentive, ready] therefore, and pray always, that you may be accounted-worthy [Kataxioo = to account worthy, judge worthy] to escape [Ekpheugo = to flee out of, flee away, to seek safety in flight] all these things that shall come to pass, and to stand before the Son of man. - Luke 21:36
And another witness, 2nd Thessalonians 3:3 has the exact same Greek word phrase in it, which in my opinion makes it another rapture verse:
But the Lord is faithful, who shall establish [Sterixei] you, and keep you from [Apo] {the} evil [Ponerou].
It is also my opinion that the very strange word, Sterixei, may have a special meaning here, as in translate, or change from flesh and blood to our eternal body. I will have to do more study to possibly confirm this.
So, back to your question.
I do not know why the martyred souls did not receive their eternal bodies and had to wait but this seems to be fact. Indeed, the English versions of Revelation 6:9 correctly translate them as souls. At what point does it say they do get bodies? It doesn't. But I also cannot find any evidence from chapter 7 that the 'great multitude' has their eternal bodies nor that this group includes the resurrected and translated people from the rapture. It doesn't indicate they are souls or bodies.
Does the fact that they have their robes indicate they have bodies? Could it be that all the people dying in the tribulation are coming into heaven as souls, including the martyrs which are separated for some reason (rewards?), and then at some point between 6:9 and 7:1 they all get their bodies?
Or, is it possible that only those resurrected and translated at the last trump get their bodies then, and all others after that have to wait for theirs in heaven for a few years, or 1000 years, as the dead in Christ waited up to 2000 years for theirs before the resurrection/rapture?
All these things must remain conjecture until something solid comes to light. I had always assumed the multitude had bodies but I think this must remain in the conjecture box until we have evidence to move it into the fact box.
I have not spent a lot of time digging in Revelation past chapter 6, but mostly reading the English, as hunting down the details in Greek takes an immense amount of time. Since I don't plan on being here then I have been spending more time on what is relevant up to the day of the Natzal/Harpazo/Rapture and what I can do to convince people to believe that we are in the very last days of this age.
Considering all these types of things is fun, to look for scenarios that will fit what the scriptures say, and every time you find new detail from digging that messes up your mental model then adjust it or find another model.
Regardless, we should never force scripture around our models and never assume the English translations are correct in every detail.