It's not as simple as you make it sound. Everybody else here chimed in with the biggest reasons.
Avgas comprises less than 1/10 of 1% of the gasoline refined in the US annually. Refineries don't like to shut down, retool and produce a small batch of unprofitable fuel which is why the other aviation fuels have gone by the wayside. Add to that pressures from the EPA about the lead content and it's becomming even more scarce. Even when Gas was around $1/gallon 100LL was selling around $3/gallon.