The Global MX-5 Cup is a race series sanctioned by the SCCA (Sports Car Club of America). Kind of weird for the SCCA to use a car that apparently isn't a sports car.
It doesn't have a lot of horse power, I get it, but horsepower is not what makes a sports car. Would you say a Decathlon isn't an aerobatic plane because it isn't as capable as an Edge 540? Would you say a T-6 isn't a warbird because it was a smaller engine than a P-51?
Like a Decathlon and a T-6, a Miata separates good drivers from bad drivers on a track. Without the extra power, if the operator messes up, they aren't able to cover up their mistake with power. Plenty of people show up on their first track day with a Mustang or Corvette and have no idea how to drive. The enter a corner too hot, mess up the line, and end up coming out of the corner at a low speed. Since all they have to do is hammer the throttle as soon as they get on the straight, they aren't aware of their mistake and spend the whole day thinking they are fast.