If the church and Old Testament saints are to be resurrected at the same time, why do we not wait for the Tribulation martyrs as well? How are they different?
The Church saints and the Old Testament saints are not resurrected at the same time. The Church saints are resurrected at the coming of the Lord (i.e., the Rapture), while the Old Testament saints are resurrected later at the end of the Tribulation, according to Daniel 12:
Dan. 12:1 “Now at that time Michael, the great prince who stands guard over the sons of your people, will arise. And there will be a time of distress such as never occurred since there was a nation until that time; and at that time your people, everyone who is found written in the book, will be rescued.
Dan. 12:2 “Many of those who sleep in the dust of the ground will awake, these to everlasting life, but the others to disgrace and everlasting contempt.
According to Pastor Armstrong's teaching, no one within a given group (i.e., the Church, the Old Testament saints, the Tribulation saints) will receive their new bodies apart from the rest of their respective group. This means that the presence of even one Church saint in the heavenly throne room in Revelation 4 requires that all Church saints must also be present in the Church throne room.
But at that time, the Old Testament saints have yet to be resurrected. In Hebrews 11:39-40, we're told that the Old Testament saints will not receive what they have been promised apart from us, which means that they must wait for the Church to be resurrected first. Pastor Armstrong used this passage in his ongoing Revelation course to establish the principle that no one will be resurrected individually...we are always resurrected as part of our respective group.