There are a bunch of reasons for tournaments not being stable.
A lot of these smaller tournaments struggle to get any funding at all, let alone enough to draw the caliber of talent that brings in viewers (and the viewers bring in sponsorship money, so it's a vicious cycle). Noukky jump-started the StriveWire Monthly Brawls with her own funds. I know other people in the scene went out-of-pocket to get tournaments going too, plus the talent worked for free (they still get practice and exposure, but neither of those pay rent).
Plus, it takes a lot of time to organize a tournament. There's lining up teams, going through the approval process with Blizzard, lining up sponsors, recruiting casters, securing a Twitch channel, marketing the event so people actually show up, and so on. There's barely enough money for prize pools, let alone the money to pay someone to organize tournaments full time. So it relies on volunteers. And when you're relying on volunteers, you run the risk that they get busy with other (potentially paid) work, or just burn out.