Lockdown, and the potential people saw in it. Relative to the other similar games, from what I've been told from a few people that have played other survival crafting games, Valheim seemed to have the most potential out of the gate. Basically Valheim seemed more polished at the start, which excited them as there's a good chance it'd become even better over time, so if you see something that's a 7/10 or 8/10 at the beginning of Early Access you'd be more invested knowing it could possibly become better and even gain a point on that scale.

Keep in mind, this is coming from people I've talked to, there could be a bias and this might not be general opinion and not why this game enjoyed so much popularity. I'd reckon a good amount of people are in this camp though, even if it may or may not be the majority. I'd imagine the amount of people who do nothing but talk ♥♥♥♥ about the game on these forums would be smaller if people didn't have expectations of it due to how strong the game is out of the gate.