You are complaining because unpopular cars are.... well... unpopular. There is nothing any sim can do to make these popular really. It's a sad fact of simracing. Even Supercars are not very popular, it was rather disappointing to manage a league with these cars in AMS back then, you'd get full grids for GT3, more than the server could handle and then just a few guys would show up for these.With over 12k online I would expect that every series is populated, but like I said, many series are dead. Lotus 49, Lotus 79, Kamel GT (Audi GTO & Nissan GTP), GT1, V8 Supercars, both Indycar-series, McLaren MP4-30 are empty all the time and most of them since years. I have a far better chance getting into a decent populated vintage race in AMS2 than iRacing even it's not on a daily basis. iRacing should do something about it if it's through making those cars 'for free' or with a different iRating-logic. I certainly don't buy those cars just to use them during week 13 lunatic races, which seems to be the only opportunity to drive a Lotus 49 or 79 online.
And then exposing these contents to daily races have a bad effect on them. Empty grids tend to breed empty grids.