Originally Posted by Red Bread
Because the Miata is the most popular track car there is, with NASA and SCCA class support and they're proven, cheap and plentiful. The BRZ looks promising, has a fixed roof and some smart engineering, but until they've been around for a decade or more, the Miata will continue to be the smarter track choice.
I haven't seen what official race support Subie/Toyota are offering for the 86, but Mazdaspeed provides so much support for the Miata (and other Mazdas) that it's a huge bonus for any aspiring track junkie.
On any given day at the track, you will see half a dozen stock to modded Miatas. The funny thing is, I've seen these guys passing up M3s, and C63 AMGs with ease on the track against supposedly "experienced" drivers.