I actually asked one of the engineering officers about this a couple of years ago. He said that it wouldn't happen at all / any time soon because the tech they use is different.
Currently, the MBs are only used for kids checking in/out of the Oceaneer's areas. Those are easy, as there are only a couple of entry / exit points. But all of the doors and point of sale systems where they use the KTTW card are on a different system, chip and frequency. The cost to replace these and the infrastructure behind them is very high.
The other thing he mentioned was that cards are easy and cheap to replace - MBs are not. If you lost your MB, you'd have to pay the $15-20 (whatever they cost) for a new one. Oh, and you can't get back into your room until you do...
IMHO, I'm just fine with the KTTW cards. Easy to slip into your pocket!