I answered this the other day over at asklemmy so I'm going to cut n paste:

Why I joined

• had a more intuitive interface

• had a better aesthetic

• had a much cooler name

• the dev seemed like a cool guy

• before federation it felt exciting like being on the ground floor of something

Why I stayed

• has heaps of cool features and functions that are easy to use, like seeing upvotes, better blocking

• I like the Mastodon interface too

• turns out the dev definitely is awesome and everything is very open

• it has a really chill community

• I still just like it more than the various Lemmys