I'm no market specialist, but I can't come up with a scenario in which that makes sense.
So, let's exercise our brains:
1) EA likes to keep franchises that sell well close to heart, so BF3 is only available through Origin. This is incorrect, because The Sims is best selling EA game, AFAIK.
2) EA likes to put everything they have on sale in as many outlets possible, so games like The Sims can achieve the number of sales that they do. This is, again, incorrect, because we all know Call of Duty sells more than Battlefield. So, if they used other stores as leverage for sales, they would put Battlefield on Steam as well.
3) EA doesn't care about other stores, since they know that Battlefield and Need for Speed will attract crowds to Origin, because the fanbase is really engaged in the games and will buy them o matter what. It's the one that makes a little bit of sense, but The Sims is their big money grabber, and they sell that everywhere. They don't want Battlefield and Need for Speed to sell as much?
They have their best seller franchise on Steam, theoretically so it reaches a bigger audience, however, the other franchises which have nice followers but could sell better when compared to competition, are Origin exclusive.
I don't fucking understand it!