It's doing well under Paizo. Well enough to ask for more money. But a change to another company will involve a hit. No matter what you do, the transition to a new owner hurts the product for a period of time. So if the product is already going to take a hit, why not move it to a different medium then if it makes sense to make that move.
You also assume the product is doing well because it's in paper format. I don't think you can necessarily make the causal connection between "print" and "doing well".
I will run some numbers as an example:
Assume the current production of these magazines makes 100 dollars a month profit total, 50 going to WOTC and 50 going to Paizo.
Assume WOTC wanted 60 a month, with 40 to Paizo, and Paizo didn't want to do that, so they parted ways.
Assume a new owner will make 80 a month, and under the new terms that would mean 48 going to WOTC, and 32 going to the new owner.
Assume also that WOTC doing this electronically instead of in print means it will produce 60 a month instead of the 80 due to drop off from people upset there is no print version, all of which goes to WOTC. 60 is still more than 48, with room for lots of error in their calculation, so it makes more sense to go to the new digital format. 60 is also the same number they wanted from Paizo, so it made sense to make that offer to Paizo to begin with rather than risk the electronic version.
Make sense now?