Lots of different cars = lots of ways to be unique and different. Also lots of cars means there is a car for everybody they can feel like it is fun to drive. Bop makes sure all cars are more or less useful even if there are differences in speed. The cars are also very quick and have lots of downforce which makes them easier to drive for many people who can't really handle high speed oversteer. And you have driving aids which also help at lower speeds. I'd even say the traction control and abs makes it even harder for people to switch to other cars. You can just stomp on throttle and stomp on brakes and get around corners. Even in f3 car that is not so simple. So for such quick cars they are pretty easy to jump in and just drive.
The cars are also known and run in many series. Familiarity is never a bad thing. And the cars also look cool and every one of them has their own reasons to pick them, be that the sound, looks, straight line speed, handling characteristic or something else. And I think sportscars is a class a cars that has always had that special global appeal even if it is on the very superficial level. No matter if you are european, american, african or asian, you know these cars. I'm not much into gt3s but when I started simracing I was definitely very into sportscars. So I'm sure gt3 cars especially have a lot of appeal for new sim racers.
The racing in general is good and there are lots of other people driving those cars which guarantees more success for the sim as long as you have even someone playing that sim online. Not to mention the simracing e-sports tend to be a lot about gt3 cars as well. And gt3 go really well together with the other heavy hitters. Spa and nords while also providing a wide track list in every sim that replicates the real world offerings. It is not just that you can drive a car that you saw in tv or in a stream in a real race or in an e-sport event. But you can get the cars, not just the car but a lot of the cars, you can get the tracks, not just a track, and you can even get the right weather. It is one of the few car classes that offers that full combination of the pieces in many sims.