The problem is there hasn't been a suitable 100-octane unleaded replacement until very recently (see: G100UL or UL102), and lower octane fuels cause detonation.

The problem is there hasn't been a suitable 100-octane unleaded replacement until very recently (see: G100UL or UL102), and lower octane fuels cause detonation. Last I checked, how well the replacements perform is still an open question. Everyone in aviation wants to get off 100LL fuel, we're just waiting for the FAA to certify one of the replacements as safe.