The IOC formally lists popularity and sponsorship opportunities as criteria
No. Really. This is the list of criteria the IOC uses when evaluating a sport for acceptance. Note the entire section dedicated to the universality and wealth of the sport, and bear in mind that virtually everywhere, from Brazil, to the whole of Europe, to India, Japan, Australia, and of course, the US, motorsports are immensely popular, and in various disciplines (more on those in a minute).
You want sponsorship? Car companies are practically begging to jump into the fray. In case you've missed it, there have been approximately 4,281 car ads since the week began. Team USA bobsleds? BMW. Team UK bobsleds? McLaren. Even the swimming team benefits from automotive involvement, with BMW's motion-tracking sensors helping Phelps, Ledecky, et al. perfect their every stroke. That's all at a huge cost, and all told, the cost of motorsports isn't that out of line with the cost of other Olympic sports, once you take training and specialized equipment into account.