Let's look closely at what the souls of martyrs are really asking for in Rev. 6:
9When the Lamb broke the fifth seal, I saw underneath the altar the souls of those who had been slain because of the word of God, and because of the testimony which they had maintained;
10and they cried out with a loud voice, saying, “How long, O Lord, holy and true, will You refrain from judging . . .
Stop right there.
The souls are asking about when
There is indeed a reason for the "first resurrection"--and that reason has to do with the reality that the saints in this verse (in bold below):
8“I am the Alpha and the Omega,” says the Lord God, “who is and who was and who is to come, the Almighty.”
9I, John, your brother and fellow partaker in the tribulation and kingdom and perseverance which are in Jesus, was on the island called Patmos because of the word of God
As we know, salvation is not complete for the believer until the redemption of our body occurs:
22 We know that the whole creation has been groaning as in the pains of childbirth right up to the present time.
23 Not only so, but we ourselves, who have the firstfruits of the Spirit, groan inwardly as we wait eagerly for our adoption as sons, the redemption of our bodies.
24 For in this hope we were saved. But hope that is seen is no hope at