I think part of the problem may be that Matrix is just pretty complex, because of its modular and decentralised design. Meanwhile, Signal is much more centralised and monolithic. And while they have added a few features over the years, its core functionality is relatively simple, and they were initially just focussed on getting that right.

I remember reading some of the pdf on state management in matrix. The math and logic behind working out what the current name of the group chat is made my head spin.