There was a couple of reasons that I switched a few years back but it really boils down to

-So much more customization for scoring. You can do 1-10 (down to the decimal), 1-5, stars or even smiley faces.

-Extremely clean interface that is way less cluttered then MAL

-Better mobile app support in AL-Chan

-More QOL that caters to the seasonal watcher

-More details about a show

Overall I think it does 99% of what MAL does but does it better. My only gripes with anilist at this point is that they don’t show the OPs and EDs like MAL does.