That's a bit silly though considering that they already have a whole Pokemon game in there which will continue to grow, evolve and become deeper and deeper. The same excuse Blizz used for that applies here too: It's optional. You don't have to learn everything about it, or even learn anything at all.
The real excuse for not being in WoW, is money. As a part of WoW it won't generate any income at all, whereas as a stand-alone F2P game they can add a store and make a substantial profit. Also given it's F2P nature it will give awareness about Blizzard franchises to people that are strangers to everything Blizzard.
And let's not forget that WoW already has a ton of distractions for people to do when not raiding/PVPing -especially after Pet Battles- so Hearthstone's impact would be minimal in the game.