Source is not easily licensable.

I follow indie games news and I don't remember seeing any recent indie release using Source. My guess is that it's no longer as popular as it used to be.

Actually Source was never a popular engine among developers. If you exclude valve we’re talking dozens maybe? And when 250+ games are released on every week on Steam it’s a very rarely used engine

When has Source ever been "rather popular"?

The only time period I can even think of that _might, maybe_ qualify would be the Source mod era, but that came and went over a decade ago. And of course they were free mods, not standalone games with purchases or licenses.

Some of these mods turned into games later (Chivalry, Black Mesa, Insurgency come to mind) but without investigating I'm not sure they even use Source anymore.

I agree that it's barely used in this day and age but worth noting that Titanfall 1, 2, and Apex Legends are all heavily modified Source

Definitely worth noting, but +3 games does not make an engine "rather popular".