ACC does not appeal to the masses. It is for players who are looking for a real sim title. GT can never be like that. It has to sell by the million. You don't do that with ACC levels of simulation. That's why GT is a sim with the sharp edges of physics rounded off.
Appeal to the masses where?
I expect on PC for current titles ACC is probably pretty high up on the popularity and sales list.
Part of that reason is ACC is relatively one of the easiest PC sim games to jump in as a new player and just go because the base car set ups are really good.
About the only thing a new player has to learn to do is adjust their tire pressures to be able to do okay.
For realism the tire model and physics is one of the best in the industry and the graphics (discounting VR) and sound is top shelf compared to about any other title.
Plus all of their tracks are accurate laser scanned renditions of all actual real life circuits used by the GT3/GT4 classes across a couple of championship series.
Also again the AI in ACC is one of the best available in any game and basically slider adjustable for speed and aggression to match the players ability. The AI is good enough in ACC you can run inches off a properly set up AI for multiple laps and never be brake checked on a straight stretch or see a stupid pace change entering or going through a corner like in GTS.
Also grid sizes can be over 50 cars.
Also add in the day/night dynamic weather and tracks that change and rubber up and grip levels and tire pressures change with conditions.
Also ACC has a much better penalty system for off track or limit cuts and car damage punishes a careless driver to where it is not unusual a punt to make a pass may well affect the cars pace unless you pit for repairs making aggressive contact with other vehicles a very possible race ending affair which seems to somewhat help curb those behaviors of an intentional nature..
Online racing and ranking is getting better with the sim grid free daily lobbies and leagues.
Really the main drawback for most is the limited car classes and overall number of tracks as compared to many other offerings.
As a player with a fairly large number of hours playing both GTS and ACC on the PC I will say just in general overall as far as game play and driving feel of the cars the ps4 GTS title does not hold a light to the PC ACC version.
Even more so for those that prefer racing offline using the AI and there is a championship season which I have never fooled with and race weekends are totally custom configurable to suite just about what anyone prefers.
Perhaps your definition of a simulation or what makes something a good game differs from my viewpoint.
All ACC is lacking period to GTS is the car variety but in my opinion in GTS the car classes are so jumbled and unbalanced and feature so many fictional made up cars within those classes it is much more an Arcade offering than an actual driving experience that relates to a realistic race experience.
But again read the features in ACC and tell me what part makes ACC a Sim that the average gamer not want to see in a driver and racing game?