As I understand it, there would be added complexity to adding UO to the EA store.
It's primarily a digital distribution store, coupled with the EA Download Manager, with single purchases. You buy a game, you then can use the EADM to download it, and play it.
UO is subscription based, with the subscription based on a seperate account system.
Although keys for games are emailed out with the EAstore, there is the matter than I don't believe you could purchase multiple copies of the same game, with the same account.
I think that is one change they've had to implement, with the multiple purchases of SA, although not being someone who regularly throws money at the store, I can't say for certain.
At the heart of it, the subscription model of UO was pretty incompatible with the EAstore previously.
Either recent unrelated changes to the store have made it feasible, or having a new product to sell has encouraged them to make such changes for UO (and future MMO projects).