It could be related to the tendency toward longer/more involved story-arcs in TV (and in movies--see the MCU), or it could be more of a parallel development.
Yeah that's what I mean - certainly it comes before it happens in TV. For example, Dragon Mountain comes out in 1993 and that point the vast majority of genre TV was still largely episodic. And Dragon Mountain, whilst a good example of an "early Adventure Path"*, it isn't the first. Curse of the Azure Bonds (1989) was the first adventure I ran which felt like later APs (it's shorter but it's not short, it had a strong storyline, and a large number of widely-separated locations). I didn't play the original Ravenloft, but did it play out like that too maybe? Like a series of essentially linear-ish adventures coming to a climax and so on?
And from many accounts loads of people's campaigns in the 1980s ran that way. Some people were running pure sandboxes with no plot except what the players made, and some were all dungeon-crawl, all the time, but some definitely has these arcing campaigns. So I think the whole thing goes back a-ways. It's not really reflected in published adventures
reliably until 2E - but by mid-late 2E, it's become absolutely the norm for adventure design.
Other RPGs were certainly doing long arc-plot stuff since the very late 1980s, I note - not all of them. An interesting comparison is Cyberpunk 2020 and Shadowrun. Shadowrun has long, complicated adventures, some of which feed into other ones, from the get-go (certainly early 2E SR), where CP2020 is much more episodic in design in terms of official adventure and so on, until the very late 1990s.
*= It does have the titular location, but there's looooooooong run-up and the location itself isn't designed like a dungeon, it's more like neighbourhood, with factions which can be turned against each other, and various possible outcomes.