[rotorwrench]

Aircraft use high octane fuel for a different reason than altitude so higher octane fuel will still work at high altitudes but it is less necessary for lighter duty automobile engines.

At high altitude, there is less air so less fuel is needed to stay in the operable mixture range. It's generally cooler at high altitude as well. Avgas is not all that volatile so that it won't vapor lock but it has a lot more expensive blend of fuel plus the use of tetra ethyl lead.

A lot of it boils down to cost per gallon too. When I lived & worked in the Rocky Mountain region, the fuel volatility would go way up for winter use. If a sudden warming trend happened, the fuel system vapor lock started to show up bad. Mountain states usually are farther from the sources of fuel and that really drives the cost up. Use of the lower price blends is more common due to that as well.