It's likely as simple as Dallas had a standing agreement with Idaho. There was no guarantee the CHL-ECHL group would get this done, and Dallas wasn't going to be left without a team. They clearly were going to have too many prospects to all fit in the AHL.
In a perfect world, I'd love to see Dallas buy Allen. At the very least, I'd be surprised if they weren't the ECHL affiliate next year. The SJ agreement is for one year, and they'd obviously prefer someone closer to them ... most of the ECHL teams are closer to them. Even Idaho would make more sense for them.