Because Marco and Star are actually well written and well thought out characters that you can get invested in rather quickly.
Its got an appealing looking art style with some really neat background art.
The show's music direction, under Brian Kim, is insanely good. The music, its sheer variety and the way its used sparingly is a huge part as well.
Besides that, I feel like its one of those shows that you'd think was awful, but if you actually watch it, you realize its got so much more to it.
Its also a heavily inspired work. The lead of the show is borrowing stuff from all over the place in anime and media and presenting it in a new package for the western world.
This show just straight up surprised the hell out of me. It appears to be juvenile but then ended up being so much more. I can't say I've felt that way since... THAT show came out.