It would be awesome to see Splinterlands on the Hive blockchain, but... I would say that JS keeps them by the balls as they have a lot of STEEM on their accounts, and if they decide to move, I suppose that he would freeze their coins as he has done with other big accounts...
Maybe that wasn't said "in loud", but I would say that some other dApps have similar thoughts... ;)
This is just my 2c...
I don't know if that's the reason why they haven't migrated yet but Yaba's comment was promising.