Even accepting it as 'popular' that doesn't mean that people enjoy watching it.

The gameplay is repetitive, and that isn't a weakness if you enjoy what it is you're playing - but watching?? That's very. very different. Doing the same thing over and over again in a game like Warframe is satisfying to do because of the highly optimised and satisfying gameplay, along with the complementary impressive visuals - the inherent 'cool factor' present in the theme, and everything you do, makes you feel like a God, and that is fun -- to play. If you enjoy the core gameplay loop than it's theoretically endless entertainment - coupled with the incentives of progression through ranks, mr and mods, you feel a pull to play on for your own gain.

But while watching, you have none of that. There's no feedback in any of what you're seeing - hell, with motion blur and colour correction the game is a mess of visual noise. But even if you do have the knowledge and experience to understand and appreciate what you're seeing you're still only watching someone else play God, and that isn't very fun. They'll make decisions you wont, and for the most part these wont have a great deal of bearing on anything, it's not a strategy intensive game - it'll come down to preference and that point, all watching does is make you want to play the game for yourself - to be God, your way.

Then there's the problem of skill, and specifically skill difference between players. Warframe is not difficult. The hardest part of the game is understanding how new systems work, the actual gameplay itself is very easy to pick up and a lot of the best weapons in the game are not mechanically intensive. They all mow down dozens of enemies at the same time, in a system that rewards time efficiency and mass stopping power. And that isn't a bad thing, it's satisfying to play, it just isn't fun to watch - but more than that, there is nothing to be learned from it. You look at other games that are popular to watch and you see League, Counter Strike, Hearthstone - why?? Because people watching are, in 90% of cases, worse than the content creator and the viewer is wanting to see something they themselves are simply incapable of doing in the game. They wont be able to pull off a .7 second rotation of Rivens combo in league, they wont be able to mindgame an opponent into surrendering in HS, and they wont be able to pull of a game winning quint headshot ace in CS:GO. They want to be impressed and experience something they can't experience in the game itself - and learn in the process. But Warframe is the opposite, by design. Skill is minimal, performance mostly hinges on prep work and loadout. There's no do or die, there's blow up 50 nerds with a Tonkor, or 60 nerds with a Tonkor, and for a lot of the high tier gameplay is that with XXX weapon. Not to mention the lack of any failure state or any semblance of tension or consequence. Worst case in Warframe, player X loses a run, he runs again, he may win or lose, and that will likely come down to whether or not he or his team brought the correct loadout. Either way, he runs again and eventually he will get the drop he's after. The drop that HE is after.

And that's another problem. Content producers would be progressing their own agendas, with their own goals - they'll make decisions that you disagree with and use a style that you wont like - all for resources you have, or don't want. Waframe gives you the tools to GET that cool thing you saw that guy have that one time. So that's what you're going to do, you're going to play and you're gonna get it, you aren't going to spend hours watching a stream of someone else get it, or a youtube video of someone else using it. Warframe gives you the tools to be God, for free. You aren't going to waste that watching someone else do it.

If there's content on warframe it'll be for new things that people need info about and tutorials for noobies, because the games are shit. Really shit. And if you do a quick search of Warframe on youtube that's what you get. Sorry I went on. Warframe is popular, but it's built around the concept of acquisition, with clear steps to get what you want. Mechanical skill and genius strategy in other high skillcap games isn't so easily achieved, so why spend time dreaming of what you could have in a game that makes it so easy to get what you want.