There are plenty other FPS games being streamed constantly and many of them gather a consistent following, bigger than tf2 infact.
Not really. FPS as a whole is a fairly dead genre, and while games like Battlefield 4, CSGO and probably whatever the newest COD is have more viewers, that's really to be expected because they're much more recent games with existing popular franchises.
I'm not trying to say as a community TF2 should stop bothering with increasing stream views and exposure, but I think there's 2 main points to keep in mind:
1.) TF2 is 5 years old. This is impressive, seriously look at the stream views numbers again and try keeping how current the game is in mind. You'll see the occasional OoT or something because Cosmo or another popular speed runner is streaming, but almost every popular game is recent.
2.) FPS is no longer one of the leading competitive genres like it was a decade ago. You say that there are other FPS with larger more consistent viewer bases. There's like 2 or 3, and they are COD, BF, and CS. That being said, it's rare for an FPS to even be top 5 games played unless it was released within the same month or two.
TL;DR I'd say TF2 has probably one of the biggest communities for a 5 year old game