Why? We've had well over a million accounts registered at jabber.org. We came up with a hacky solution to speed up the migration: we initially imported all accounts that have been active in the past 6 months. Now we're importing them on an on-demand basis. If you attempt a login and your data has not been migrated yet, you'll get "account disabled" error but your account will be prioritized for import during the next migration run (which is now running asynchronously multiple times per day).
Also note that the migration this weekend was just the first step in bringing the service back to life, it's still lacking a number of modern features that are widespread elsewhere in XMPP these days (for example, no push notifications for mobile clients). We'll be turning these things on over the coming weeks.
Finally, once the dust settles, we'll be looking towards the future of the service. Potentially opening up registration again (closed since 2013) and options to help the service be more self-sustaining (such as accepting donations).