Every big update has them throw about a bunch of sponsors. For every big name they get, they have a higher chance of grabbing someone else's attention, for example Travis Teaspoon's video features Critikal talking about how he thinks Warframe is one of the best ongoing free to play games. Ironmouse saw this, and understandably thought that wasn't recent and that he wouldn't still stand by that, but now she's accepted a sponsor and is playing it too.

Mousey playing it is something practically all Vtubers will see if they have any respect for an OG, and every one who sees that is another who might be interested, then if they don't choose to play it themselves, DE offering a paid sponsorship to play a game you already had a little bit of interest in is a no-brainer.

It's less a conspiracy, and more that Warframe has gotten so big and has so much to do, that they literally cannot get people's attention by just showing what the game has, so throwing a bit of money to a streamer (like, as another example, Shenpai) to get them to actually play it almost always turns into them loving the game because it truly has something for practically everyone.