People always tend to gather around the fireplace that warms the most. iRacing having more people joining for organized/"professional" racing doesn't necessarily mean it's the best.I always like to check out these threads to remind myself how badly iRacing sucks, because the 100s-1000s of people I see racing on the platform at any given time of day makes me wonder sometimes if iRacing isn't doing a lot of things really well.
Whether it's good or bad as a simulator, it's relative, in a sense. If you want to play online with other people, you look at charts and go to whatever service has the most players connected, but that isn't necessarily an index of quality. It just means it's more popular.
It's like the thousands of people playing GT Sport or Forza on console. Do they do it because they're best games? Not necessarily, they do it because they're more popular and more populated than the rest, otherwise they would go to the alternatives, like some other so-called simulators available on console.