a reason for Hive holders to keep their Hive off of exchanges and a way to earn some staking rewards while providing a useful service to the network

We already have that. You earn about 3%/y staking as HP even if you ignore curation, relative to keeping it on an exchange.

There has to be something about that people don't like, or more would do it. I don't think it has much to do with which token is being used to pay out the 3%.