To be honest, as someone who has ridden bikes since the early 1980s, and started primarily as roadie -- "gravel bikes" are succeeding because their geometry is more relaxed than trad road racing geo, their tires more cushy, their gear range wider for un-fit cyclists new at the sport. And during this same 40 years infrastructure needs are largely ignored in many parts of the nation, roads are pockmarked or potholed, traffic is crazy.
The "gravel bike" is more comfy on poorly maintained roads, more comfy for someone without 100s of hours in the saddle in road race position, geared easily to get you up a grade, and can be ridden comfortably on secondary roads where there is less traffic.
I don't think it comes from enticing roadies to be MTBers. Though that is how the cycling media, especially MTB cycling media, have pitched it. "One big club" and all that. Hey man, cyclists aren't a subculture that way, and it's goofy to use a bicycle as your "in-group" identity.
Why a "gravel bike" and not MTB for a new recreational cyclist? Efficiency, mostly. The more efficient the bike feels in the setting you use it, the more likely you will keep riding. The more it feels like hard work, the more likely you quit cycling and it becomes a dust magnet. A "gravel bike" is more efficient at city errands, road rides, country dirt/gravel road rides. You can ride singletrack MTB trails on a curly-bar "gravel bike" even.
A mtn bike used mainly on the road, or gravel/dirt secondaries, feels less efficient if you compare them directly.