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.
It would be direct integration (maybe I chose the wrong term) if piefed could share the database with Lemmy, then uses only had to start using the new features in piefed.
Because they are two different activitypub implementations I would not expect easy migration to be possible in the near future or ever, most likely only if it becomes a part of activitypub.