Well, I think that each and every one of us is most likely influenced most by the decade of racing that he grew up with. Being a 70s kid, that would obviously be the 70s, 80s and 90s for me. So anything before that feels "old" and may be like "yea, dad, tell me more about the war and all that". However, my personal interest has always been the older things, in almost every aspect of modelling and general interest with the most modern car in my stash being a 934/935, same with planes, exclusively WW1. The general public is most likely to go for what they experienced first hand - see above - so the young 'uns will probably go for cars of drivers who they've actually seen racing.
That aside, depending on scale and budget at hand. A lot of the kits named in the initial post are available (in 1/12): Ferrari 156 (MFH), Alfa 158 (MFH), Alfa 159 (MFH), Lancia / Ferrari D50 (MFH), Maserati 250F (MFH). And those are just the ones mentioned in the post. There are other cars like the 500, the 801 (in 1/20), the 256 F1, the AM DBR1, the Alfa P2, the Bugatti Type 35, 250 TR, 315S / 335S (all of these from MFH). Then there are small specialist manufacturers like Fernando Pinto who has a 375 Plus, the AU Type C, the 625 F1 and 375 F1 (all1/24) and I believe I saw a W125 as well somewhere...
So I think the models are there - well not all of them obviously, but still. And as for the why - wire wheels have been cited and I absolutely agree. Just think back to the MFH wire wheels and the amount of effort it takes for just one of those using a jig! And to have that properly represented in a model without losing too much detail or turning it into a giant blob of resin is a lot of work and may come out of casting with a low hit to miss ratio. And then again, there's the focus groups (per my first paragraph) and the resulting promise of sales or rather lack thereof. And since most people are trying to run a successful business, sales are what matters. So the 431st reiteration of a Senna car will almost certainly beat a Moss car any day.
Don't get me wrong, I LOVE the old bits and am considering myself lucky in that the '67 F1 (GPL?
) season is well represented, but I would appreciated more of the old cars and we're lucky that Hiro san and Fernando Pinto (to names just those two) have given us plenty of models (see above) that from a pure sales perspective were probably DOAs to begin with.