The build order specified is usually to account for minute errors in layout and hole drilling. The reasoning goes like this:
The trigger guard holes are usually set, and the trigger guard contains the mag catch assembly. Once that's in place, you can slide the front trunion in and adjust it's position up against a magazine until the mags catch and move freely with the mag catch, which is now fixed in place. Then once the front trunion is set in place relative to the trigger guard / mag catch assembly, then you use the dust cover to set the distance length for the rear trunion, which has the most available movement/"slop" in it's final position from the front trunion. So it's kinda working your way along to make sure one piece lines up off another.
If you were to set the front trunion and rivet it in place first, it may be too far from the mag catch/trigger guard/magwell for everything to line up and function properly.
Make sense? Anyway, that's the reasoning behind the build order logic.