And they're expensive
Modern 'sports/road' tyres like the BT090s are really good these days, and tend to work over a much wider temperature range even though they don't offer the absolute grip levels of race tyres; they're probably a better compromise for most track use than proper track tyres these days.
*Many years back I got invited to a race team's winter test, with a go on one of their their race bikes in return for the coverage I'd got them in the press over the previous season. I was sharing a bike with another journalist (who was a proper racer), and when he came in one of the teams technician's checked the temperature of the slick tyres before letting me out on track.
When I came back in five laps later, he checked the tyre temperatures again, then looked at his gauge, rechecked them, tapped the gauge, checked them again.... He couldn't believe me riding as hard as I could (on an unfamiliar circuit, to be fair) had allowed the tyres to get so cold...