Here’s another one, by  Telebear:

One of the biggest issues standing in the way of TF2 becoming a successful e-sport is not to do with sponsors, lans, or numbers of players, but rather its ability to be a game that is able to excite and entertain spectators, do this and the rest will follow.

Simply put, TF2 in its current format is a boring game to watch. It does not posses any of the qualities that makes other games an exciting prospect to watch, it does not have the speed or match pacing that a game such as Quake has, nor does it have the deep strategic depth that makes Starcraft so watch able.  TF2 is simply too slow and not in depth enough to ever be a spectator game in its current form.