Steem started out as a decentralised blogging platform. Recently it’s main interface (Steemit) and all the Steem currency that belonged to it, was bought by a rich Chinese crypto troll (Justin Sun) that is now destroying it. The original community that started Steem build (“forked”) a new platform to save their vision. You can login to Hive with your Steem keys (if your Steem account already existed before Hive).

Tldr: Steem became centralised dogshit so we all went to Hive.