why some subs didn’t migrate to a Lemmy instance

Some subs are definitely doing it, check here: https://sub.rehab/ and here: http://www.quippd.com/writing/2023/06/15/unofficial-subreddit-migration-list-lemmy-kbin-etc.html

Some subs will need more time to organize probably, some will probably stay on reddit, but that's ok, it's users that have to move first and foremost, power users and content creators specifically.