Well at the polynesian they have 2 queen beds and there is a sofa?
And some Disney resorts have two queen beds.
Some have a queen plus sofabed.
Some have a queen plus bunk beds.
Some have two queens plus a daybed.
Some have two double beds.
Configurations vary greatly from one Disney resort to another. There's no reason to think that any one resort would mirror another. In each case, it is what it is.
Old Key West was the first DVC resort and it's the only one with the two queen beds. They've since built 9 more resorts with the queen + sofabed configuration so the logical conclusion is that more guests favor this setup than are opposed to it.
The presence of a kitchenette in Studio villas has nothing to do with the sleeping accommodations. The kitchenette is simply a couple wall-mounted cabinets, a mini-fridge and a countertop. It occupies about 2-3 sq ft of room space.
It's all about flexibility. Some groups don't need two beds and the presence of the sofabed means that half of the room is actually useful rather than occupied by a bed that will never be used. Meanwhile, those who do want two beds can simply unfold the sofabed.

