The creator of the sub is heavily biased towards kbin, so they restricted r/lemmymigration to sway people towards kbin. They don't seem to be willing to give the subreddit to someone who actually cares about Lemmy either.
Yeah it's very unfortunate, those subs could bring alot more people to the fediverse if they weren't so biased, specially with the fantastic apps Lemmy has already.
The Lemmy subreddit directed people to Lemmy at first, but after the creator was made aware of controversies surrounding the developers and lemmy.ml, they decided to restrict the Lemmy sub and forward people to the kbin subreddit instead.