Well, turn based strategy is a pretty niche market. Mmos cost lots of money to make, so if you're gonna make an mmo you generally want it to appeal to a wide demographic, so it will make all that money back.
There's also the problem of massive combat, where you have 40+ individual units fighting at once. That means you take a turn, then 40 other people take a turn, then you take a turn. That's a huge wait time that most people wouldn't tolerate.
Mmos tend to have lots of enemies running around, but the enemy AI tends to be pretty simple. Run towards hero, attack, use abilities. The AI for a turn-based strat game like Divinity needs to be much more complicated. Running lots of complicated AI at the same time would be very hard on the server.
That said, there's no reason you COULDN'T make a Divinity OS style mmo. In fact, you can technically do that with Divinity OS with mods. People have already made mods that
-create huge scale battles
-allow 4 different people to play together at the same time
So just combine those and you have a theoretical mmo.
The netcode is already buggy, though, so I doubt it would run particularly well.