Because it's throwing the baby out with the bathwater. Some of these mods were literally about making the game accessible to people who wouldn't be able to use it otherwise. There's no way they're going to keep up with all useful mods with their official client. Some of their concerns are valid, but at the very least they should provide an option to allow modded clients on your own private instance.

Unless they manage to make VRChat mainstream (which will likely erase a lot of its appeal to plenty of current users and will mean directly competing with Meta), this may be the beginning on its slow end.