I have worked on an RFP for migration. It persists because the cost for migrating can be quite high -- many mainframe environments have many applications with complex sets of dependencies (after decades, not surprising). No one doubts that it could be migrated, but it is not a trivial undertaking.

When folks have replaced significant enough chunks with other apps, or otherwise made the prospect less daunting, then it may happen. It just comes down to cost and risk management.