The problem is you can't just give out Magic Bands and that's it. There's a huge rollout involved in terms of infrastructure. Essentially you need to upgrade every room to include locks that accept the magic bands. You need to upgrade the turnstiles and fast pass machines with magic band readers. You need to equip all photopass photographers with magic band readers. You need to upgrade all retail and dining outlets with magic band readers.
the whole appeal of the magic band system at WDW is that it's a one-stop-shop for all your needs. If you have it on your wrist, you don't need your wallet, room key, photopass card or ticket. Offering it JUST for fastpasses would undermine the value of the system, so IMO if you're going to roll out magic band, you need to do it completely across the board.
I say this as a huge fan of the magic band system. I would love to see it at DLR. Just realistic in terms of the amount of work it would take.