I don't have demo numbers or marketing survey results, so I can only offer semi-informed guesses regarding the US market.
- The majority of motorsports fans in the US follow NASCAR. Most of those NASCAR fans like ovals better than road courses. Most of them like more beating and banging than is permitted in sports car racing.
- We're in an era of where sports fans aren't as willing to watch longer events as they used to be. While six of this season's 11 IMSA races are under three hours, the other five (and all eight of the European WEC series races) are at least six hours. They're too long for a single US network to cover in one broadcast window, so watching some races requires switching networks and streams.
- With races featuring multiple classes, dozens of cars, and hundreds of drivers getting in and out of cars, it takes more than casual attention to keep track of who's who and where they're running. It requires some practice to learn to keep track of their strategies and how they change as the race evolves.
On the other hand, attendance has been growing for the last several years, barring COVID. Tickets in the US are relatively inexpensive compared to Cup and far less than F1. Garage access is unparalleled. Weekends usually feature multiple lower-class races. There are few full-course yellow flags thrown to interrupt the racing; local yellows are used while cars are given more time to recover. The number of participating manufacturers is growing in both IMSA and WEC since the sanctioning bodies made it easier for teams to run in both series.
And the Miatas are a hoot and a half.