the problem for me is the amount of players in MP. in the first days i remember i raced vs full grids every time, and now its even hard to find one with atleast 5 players in it, this means the community got disinterested in it. and if people are not interested in it anymore, you can develop the sound/graphic engine what you want but its not gonna help, because after all its not about SP experience, its about the resources in MP that keeps people interested. thats why it annoys me they work on the things that are not primary atm but secondary.
Well, I think you are conflating two different things here and I don't want to single you out, the following is meant as an explanation for others as well.
First is player interest
Player interest, the participation rate in our club races at the moment is a case in point, has dropped significantly in the last few weeks, you're right about that. Nevertheless, it is summer time and the World Cup is going atm, you have to keep that in mind.
A further reason is i think a symptom of current player culture, and that is to buy a new game, preferably one on sale, try it for a limited time and then move on to 'the next cool thing'. Assetto Corsa is certainly not equivalent to the abomination and 'Pewdiebait' (Trademark Jesse Cox) that is Goat Simulator in that respect, however this player behavior is now more commonplace then ever. I think nowerdays gamers lack the patience and stamina to stick to one game for an extensive period of time and
then move on to the next one. Look at the steam profiles of your friends and/or other steam users. Most likely they will have one or two games they'll be very into, with hundreds of hours of play time, but most won't even have a handfull of hours played and many games won't even be started by them at all.
I think the appreciation of being able to buy and enjoy games in general has dropped in recent years, especially for prolonged periods of time.
The second one is marketing and commercialisation related
The marketing budget for movies is absolutely massive and oftentimes equal to their complete production costs. The marketing budget for tripple A titles, and hyped games like COD, is in the hundreds of millions as well. The genre and targeted demographic is what differentiates Assetto Corsa from other games. Just being a racing game, AC has naturally not the breath of audience then a FPS for example. In addition, AC is a racing simulation, what further narrows down the intended player base and Kunos is not a big development studio with a giant publisher in their back.
Assetto Corsa is still in early access phase. While this gives players the opportunity to play the game before it's officially released, it's not the right time to advertise and long for a bigger playerbase while this game is unfinished. I think AC has 'beta testers' enough for this purpose. Kunos will be in a stronger postion to make their case for AC, if it is fully developed.