Cruise speed.
The hub sort in KMEM and KIND is based around the speed of the 727. If they were replaced with the 73 or Bus the outstation departures would have to be earlier. Therefore, the delivery trucks would have to finish their routes earlier - and that would cost money.
I used to do MEM - Boise, Idaho in the 727 at Mach .85, and when we'd land the outbound crew would be standing there waiting to take it back for the day sort. No time to waste.
As for the planes, I cannot believe they made this announcement without the aircraft already identified and on contract. Otherwise the price would go up. But I haven't heard where they are yet.