Whizbang Dustyboots
Gnometown Hero
I think the fact that she's a known quantity among engaged 5E DMs, due to her very well-regarded adventures, helps a lot. She's spent years garnering a reputation as a good writer who understands both mechanics and theme very well. (Even her lightest 5E adventures have a horror edge to them, and this is her going harder in that direction.)However I think it's success shows that clear communication, marketing, good presentation and layout and strong support are highly valued in the market. While there's nothing in the rules that particularly wows me, there's nothing that turns me off either. And you can't deny the product is put together well and the campaign has been run very professionally.
A lot of OSR folks are known, if they're known at all, inside the much smaller OSR space. Being even a medium deal in 5E land gives her a huge edge over all but the best known OSR folks, and maybe even over them.
And yes, since she's a YouTuber, a lot of the other YouTubers know her already, and the ones that don't, as always, are hot to chase whatever's getting clicks, now that yelling about the OGL has stopped being lucrative. So she got the ball rolling by reaching out to a few of them early and things picked up speed from there.
Most of that can't be replicated by generic "marketing" -- it's unique to her. She put in a lot of work and is reaping the rewards.