I did a poll and nearly 30% of the people who voted said they used Sync! It's stayed around that percent the whole time too!
EDIT: Here is the poll I forgot to put this here like five times lol https://strawpoll.com/wby5A21R1yA
Privacy. Sync includes Google trackers, whereas other apps for lemmy do not. I get it though, gotta make money somehow. Still, I'd rather use something else and donate to the dev on occasion.