At BlizzCon every year, Blizzard puts on a SC2 tournament, a WC3 tournament, and a WoW arena tournament. The SC2 tournament is well attended most of the convention, and packed for the big matches (as in, you're not getting close to a chair unless you've been camping one all day). The WC3 tournament would be fairly sparsely attended if it weren't in the same place as the SC2 tournament, so people will be camping out there and either lamenting how boring WC3 is to watch or getting someone to hold their seats so they can go get some food. The WoW arena tournament is... not well attended; there were chairs available for the final (which last year went past the "closing ceremony" so there was literally nothing else going on).
There really is something about a game that makes it more or less watchable. WoW arenas are not all that fun to watch because there's a high density of important things happening at the same time, so it's difficult to understand as a spectator, and unless you're already a fairly high-level player you have no idea what those things are, so that locks out most of your potential spectators. On the other hand, a bronze-level player can understand what's going on in a pro-level SC2 match, even if the pro is chugging along at 200 APM.