I think the top 3 reasons are, ultimately, the same reason; the people who are already there don't want you there, and they like the obscurity of discovery and obfuscation of communication, confusion around instances for onboarding, and ability to gatekeep exactly how you're allowed to use the platform.
There's issues with the underlying platform, for sure, but the established user base likes it the way it is, and is very strongly invested in preventing change.
And, that's okay! If you have a platform that you enjoy using, it should be defended, and aggressively.
But, at the same time, you shouldn't be utterly confused why so many people either don't want to or bounce right off your platform and aren't sticky when it's pretty obvious (and has been for a while) that the culture is the big driver for it.