I went hardcore into TF2 competitive for about two years like a lot of other people here, silver scout for a few seasons.
Unfortunately the TF2 scene dug itself a grave with its metagame
6s is fast-paced and and exciting. But there is no meta-development, as there are tons of weapon bans, lots without valid reason
HL is boring and campy, but it embraces weapon customization. Unfortunately 9 players makes for more campy games than 6 player games...
In other words you either have a game that's fun to talk about but not fun to watch, or vice versa.
Another issue was a lack of personality. Some top players like Taggerung were a bit more vocal and made people interested, but for the most part there aren't any Mang0s or Leffens or M2Ks or even good commentators for that matter. The commentators are even more bland than League esports, even though there are zero big name sponsors in TF2