VRChat came out more than 5 years before Horizon Worlds did, and already had an established player base by the time Horizon Worlds even came out. It was in early access for ages, but that didn't matter to the VR audience, who's used to there being jank. Not to mention that Horizon Worlds was probably never more of a finished product than VRChat to any significant degree.

If you've already got a VR social space, why switch to a different one?

VRChat was designed for people to hang out, Horizon Worlds was built as a platform for people to purchase meta services and desired corporate use which came with a lot of sterility (you don’t want half the people in your corporate all hands to be some form of anime girl, see the reactions that even just Anubis’ logo gets here).

This came in both world design and novel design where vrchat lets people import basically arbitrary 3d models with a wide range of expressiveness. VR went for corporate Memphis upper torsos which immediately makes the world feel more bland. Letting people import whatever would also eliminate the opportunity for meta to sell that, and have the risk that there might be too much breast for a corporate audience.

Add to that and the fundamentals just worked so much better in VRChat when horizon worlds tried to make its big splash and the fate was sealed.

Porn, for one.