A few years back when censorship was really ramping up, and people were being punished or banned on the major platforms there was a big push to adopt new "free-speech" platforms.
I became aware of Steemit because of the The Dark Overlord group posting evidence of a 9/11 conspiracy they had hacked from a law-firm's digital storage.
I think that probably got a lot of people onto Steemit, and then the Steemit Wars happened and it became Hive.

But the other major push was the explosion of crypto and NFT's.
Everyone saw this as a digital goldrush, and so you had lots of people trying to find crypto that had utility. I remember DLive hired PewdyPie to stream there exclusively.

Rumble seems to be growing.
But I've noticed that many of these platforms are a shadow of their former glory.
Minds.com is another one that's crypto-based, and has become like a ghost town.
Odysee was the better video site in my opinion. In terms of tech, and lots of other things. They got forced into a sale because of an SEC lawsuit they lost.
I doubt they're growing.

One thing Rumble has that these other sites did not: Major funding/investment.
Peter Thiel, co-founder of Paypal with Elon Musk, has a major stake in Rumble.
And then the next one would be TikTok, but I think it took off in Asia first.
Hive on the other hand arose in a war against a billionaire.