Here's my two cents:
Because there's no direct integration with Lemmy, and we start with an empty piefed instance. I'll have to create a user and all that, and I "just" quit Reddit so it's not very appealing to me, even if piefed has better features.
I would create an account if everyone migrates, or if I can click a "move to piefed" button that copies all my comments and posts.