Project 64 1.6 is already very old and clunky.
Why is something like Mupen in Retroarch not allowed?

mupen64 plus is allowed in the rules

mupen is not allowed because of its tas functionality and I have never heard of retroarch so it is possible that mods have not either

Straight from the rules for emulator:
Allowed Emulators:

Project64 v1.6.x
OpenEmu (for Mac users)
Mupen64Plus (for Linux users)

Retroarch adds a performance increase and is considered unfair. mupen64 is for tasing, and mupen64 plus is harder to set up, and even more clunky in my experience compared with project 64.

honestly, the mods probably should look into more emulators like bizhawk, mupen64++, and others. yes, bizhawk is used primarily for LOTADs, but it's actually more accurate than the supported emulators. as for mupen64++, I can't remember if it's android only, but I find that it's the most accurate, assuming your device is powerful enough to run it.

other mac emulaters should be allowed because openemu is for macos 10.14.6 like sitcyforce and pj64 1.6 is old they should add 2.0 for speedruns

i mean, mods are not allowing every emulator posible that havent been proven ilegal for x reasons yet, is kinda the reverse thing
those random emulators that each one is purposing its own, i dont doubt they probably work well, but just havent been proving yet
mods will have to check them and decide that they are legal for speedrunning before allowing it
it would be cool if they did it more ofter with more emulators

If some emulators are banned its because they arent accurate, they can be easily used to disguise cheated runs or the moderators dont think they should be used for speedruns

retroarch isn't an emulator in and of itself, but instead a frontend for emulator cores. i've submitted using the mupen64plus core for retroarch on linux.