The obvious answer, of course, is that they think they can make more money by not doing so. In order for a lifetime subscription to be profitable, the player that buys it has to either stop playing before the subscription would have ended, or not be a major drain on the system after their subscription becomes "free." It's a gamble on both sides.
Now, Tobold's reasons for liking his LoTRO lifetime subscription and not jumping on a Hellgate London subscription aren't really applicable to WoW-- HgL is going to be completely different, with no subscription fee required for some players. But would you invest in a lifetime subscription for Blizzard's game? There's little chance they'll offer one now (usually a lifetime subscription is offered at the start of a game's life), but you never know-- if reception to WotLK isn't great, maybe they'll offer it right before the expansion to try and nab players that would quit right afterwards otherwise. By Hellgate's prices, a lifetime subscription to WoW would be $225. Let's go a step further and say that because WoW is so popular, Blizzard would charge $250 for a lifetime subscription to the game, approximately the same price as a year and change paid monthly. Would you take that deal?