Because when you search for a community on kbin, you can join communities from the search results. As far as I can tell, in Lemmy, you have to click into each community in order to join, so building my list of subscribed communities on Lemmy will take a while, especially considering all the server load problems Lemmy instances have been having with the influx of new users.
For me Lemmy is more accessible so i use it even that i know that kbin can not only see stuff from lemmy and kbin, but mastodon too (at least following people is possible)
So kbin has more features. As i was testing it there where some conversations about people switching for the better algorithm.
I think kbin is in general (with features) better but i like lemmy more.