I heard heard stiffness was a factor. A smaller frame will be stiffer than a larger frame because of the shorter tubes. There’s apparently only so much that can be done with layups at these larger companies to make the bigger bikes stiff enough and even then they’re still more flexy than their smaller counterparts. Remember the world tour (generally) has to ride stuff that we can buy off the shelf. They can then put on whatever custom stem and seatpost combination they please to get the position they’re looking for. I think I remember seeing a video on GCN where they were looking in a world tour mechanic’s tool chest and they had stems in 1mm increments instead of 10.
Speaking of custom builds for larger frames, I just took a tour of the Parlee shop in Beverly, MA and they were doing a custom build for a New England Patriots line backer in which they had to re-think even their stiffest layup to make a bike to support this 325 lbs 6 ft 7 in guy (if I remember correctly, but in short, huge). The head tube on that thing was about as long as my forearm. That might be a fringe case but it doesn’t seem to be the easiest to get long tubes properly stiff.