No time to delve deeper, so just a quick glance at the numbers - The B-17 used to have around 25% of (k+d) of all heavy bombers (based on 3 sample tours in 2024, 2016 and 2009), last tour it had been 19%. Which would support your observation.
My made up explanation:
Since player numbers went down, the gameplay is getting less varied. More emphasis on base grabbing, less on secondary missions because you don't have the pilots to spare for that. For example back in the day, I was using minor bombers like G4M for porking bases along a section of the front. Or i was shutting down hangars at a key point. Both of it now being basically pointless with the shifted gameplay dynamics. When there are like just 8 active players on my team, I'm usually needed urgently for defending a specific base or a factory or be the one striking a key asset. Also, often more frontline bases than players.
So it's a lot about efficiency. You still need the same amount of bombs on target, but you have less players. The B-17 is better at getting to high altitude than the Liberator and Lancaster, but with lesser payload than those two. So it's better for the ultra-high strat runs, while the latter two are much better for quick low altitude strikes on towns (which they actually can whiteflag on their own).
You also don't see other planes as much any more, their numbers dropped of even when seen in relation to the population number. Hurricane I, Ki-43, Bf 110G. All, in my opinion, victims of changing gameplay, which is in fact to a huge part a result of lower numbers overall. (I once had a tour using the Hurricane I as my main ride - that would simply be impossible today)
