I don’t think it’s down to lack of popularity or any kind of skepticism. We all know licensing cars is a certified nightmare for all involved. Maybe it’s current relevance ? Don’t get me wrong, i’m not saying nobody cares about it. It’s mostly down to what’s more being talked about more.
F1 currently is, for me, a very negative environment to be in right now. A lot of bad decisions taken the organizers (don’t get me started on Spa being a rotation track) and the FIA, overall fandom doesn’t have a lot of respect for some of the drivers (as a Max fan, this gets even worse for me) and the racing is… Lackluster most of the time (this is a car problem mostly).
Sportscar racing meanwhile feels, as a fan, so much easier to breathe in in terms of the quality of the product. International talent that’s as good or even better than F1 drivers, equally cool machinery that gets more manufacturers on board by the year, top class tracks and, more importantly, amazing racing action!
I might be sounding like an anti-open wheel enthousiast right now but that’s very far from the truth. F1 was and is still the basis of my motorsport menu, and despite it’s questionable management, i still tune in to all of the races to support my drivers (especially Max). Racing the open wheeled cars in Forza has become a lot more fun in the current game with how it handles. Formula Mazda ESPECIALLY was peak when they started introducing oval tracks in the circuit rotation (going 4-wide at Indy for the lead was arguably my best overtake ever). Formula 70s was not as good but still very enjoyable, and i really hope we get another cycle of these cars in multiplayer. The 90s era cars i tried once : too fast to race with and more of a penaltyfest than a race (still awesome cars though).
Now, about the Formula 70s cars… When the community choice vote for multiplayer comes up, that series tends to sit at the bottom, wich also means to me it might just not be relevant for current players or other, better, series are available to vote for. Real shame to honest because… This is kind of the only place on Xbox at least where we can race these cars in an online environment. Same for Indycars as you can, officially, on race them on something like Iracing with the current batch of games and simulators. If we were to get those back, i’m sure this series would be an instant permanent lobby!
Do i think we will get more open wheelers in the future ? Maybe. There’s always the chance they do an Indycar month someday and with May being the month where their crown jewel event is held. They like to do similar things for other big events like le mans and the 24h of Daytona on Forza so that would also give them another excuse to push for this kind of update, though that probably would be after the teased Porsche Motorsport icons update in the January notes.
Personally, i think it would also be nice to have something like the old F3 cars to play with (2 gens earlier than the current one for this year’s FIA F3 season) or, even better, Japanese Super Formula cars!
Check the voting page here: 
Car Voting - FM - Official Forza Community Forums
I love all formula cars