Mostly the better documentation (last I checked FastAPI docs felt more like a series of blog posts than actual docs, but maybe that's improved). I also preferred the community-driven approach of Litestar as opposed to FastAPI's BDFL-type development structure.

I think there were also some technical details I liked better about Litestar where it was more explicit about things while FastAPI was more "opaque magic happening in the background", but to be honest I don't remember all of those.