Wrathmage, two reasons:
1. As earlier stated, Free > Not Free. A lot of people downloaded the beta just to give WoW a try. Not all of them would have gone on to decide to play at retail. Most of us, I think, saw the open beta as a "shareware" period rather than an actual beta-testing period.
2. MMORPGs usually aren't as popular as single-player or LAN-multiplayer games. The MMORPG (or just MMOG) market is widely considered to be nearing or already at saturation point. Notice that most current WoW players are ex-FFXI players, or ex-EQ, or ex-DAoC, or ex-SWG. WoW has the potential to lure in a lot of new MMOGamers, but the truth is that it is difficult to expect any new MMOG coming out now to be as wildly successful as any non-MMOG. The fact that WoW has done so well is truly a milestone, and every other MMO maker now has a much higher standard of competition (and hype) to deal with.