They have to abstract multiple db paradigms into an API that works for many use cases but doesn't get you all the way there. Eg: Last time I checked they didn't support listen/notify on Postgres.
Also overall performance has been lacking which I suspect is also a symptom of trying to support too many dbs. They recently released v6 and made claims of performance improvements but afaik offer no data to back up these claims [1]. It's all very "trust me bro it's faster".
Drizzle still hasn't reached 1.0 but they created a much better ORM in a fraction of the time (and probably budget).
[1] https://www.prisma.io/blog/prisma-6-better-performance-more-...