While that's part of it, it's definitely not "just" that.

Sadly, part of it is that the game has released in a fairly stable/polished state, which is considered a positive in the world of broken releases. The multiplayer also just works with little issue as opposed to some problems of yesteryear.

There's also a perhaps surprising pent up demand for good co-op PvE focused games. They blow-up hard but tend to fade out depending on gameplay quality. Part of this is the streamer effect, streamers like to play group games with other streamers because it helps cross-pollinate their audiences. Sales are also improved due to group/peer-pressure, if someone can pull in their friend group, that's a lot of sale multiplication.

I also think that the developers tried to make a game that's fun. A lot of decisions seem to have followed the rule of cool for this type of game e.g. pal mounts, firearms, catching people, automation of survival elements via slavery.

It also manages to have both a clear and guided progression system while maintaining the freedom for the player to just fuck off and do whatever they want while still at least partially progressing.

My only honest gripes with the game are how world saves are handled (they should use the Grounded system in addition to having dedicated servers) and that I for some reason can't find the exit button on the title screen so to quit I need to alt-f4, for the rare times I need it.