Wrecker
There are a wide range of aircraft that can't use 91UL ranging from the humble Cessna 152 to the big turbo charged twins, without a 100LL replacement fuel these aircraft will be increasingly expensive to fly.
The big problem from the supply point of veiw is that you can't mix the leaded fuels in the refinerys with unleaded fuels resulting in having to have a refinery dedicated to 100LL. If you can make a 100 grade aviation fuel without the lead you can use any motor fuel refinery to make the base product, and so keep the cost down.
100UL would also be less harmfull to the environment but this is not a big environmental issue as AVGAS makes up such a small proportion of the hydrocarbon fuel that is burnt.