From experience moderating on Reddit, user histories were pretty useful in judging whether they just made a mistake or were ban evading or trolling. If a fresh account drops in with a trollish comment as their first interaction with the community, they might just catch a ban rather than being treated as a good faith poster who came in too hot and deserves a second chance.
So if you migrate accounts in Lemmy, you’ll have to pay that price over again and risk more strict moderation because you have no history, whereas a Mastodon-like link to their previous account would establish a baseline.