Because Eugen has been working on Mastodon much longer. It's a lot more mature and it had time to get a bunch of real world testing, not to mention withstand some DOS attacks, some hateful trolls, etc etc.

So pretty much every aspect was more mature and easier to use when people started jumping to the lifeboats from Twitter. Lemmy is cool and it's getting there, but it's got a ways to go for sure. (The lack of RES hotkeys to navigate, vote, open and close pics, follow links, jump between all and subscribed is purely torture to me at this point!)