Likely because they are inexpensive and are beloved track cars. I was hoping the GT88/BRZ would oust the miata from this position- I much prefer a hard top over a convertible, but I think the existing after market and price point make it hard to compete.

I think there are bolt-on hardtops for NA and NB Miatas. Looks kinda funky.

Also yeah, the Toyota 86 family is still too expensive to be a project car for most.