There are many reasons, ranging from not wanting to leave already established communities, Lemmy and Kbin aren't quite as robust as Reddit is yet, and a lot of people just frankly don't care enough to switch. Plus the lack of a mobile for Kbin doesn't help.

Just gotta be patient and win people over slowly when they willing to make the jump :) We may see some more once the third-party apps shutdown at the end of the month.