The specifications for UL 102 are described in this Wikipedia article. The fuel appears to exceed the specifications of 100LL in octane number and energy per unit volume although the fuel weighs 7 pounds per gallon instead of the 6 pounds per gallon for 100LL. Is the additional weight the reason the FAA rejected this fuel as an 100LL alternative?
The problem is basically bureaucratic delays, helped along by easing of EPA timetables on the banning of leaded fuel, taking some of the pressure off, which has caused the developer of UL102, Swift Fuels, to suspend its 2nd Phase testing until the machine gets moving.
AOPA article on unleaded fuel from Oct 2018.
The FAA has re-evaluated its Piston Aviation Fuels Initiative schedule, reaffirming its commitment to develop a replacement for leaded avgas but delaying expected testing completion from December 2018 to mid-2020.
The new timeline focuses on testing of an unleaded fuel being developed by Shell—now the lone participant in PAFI following the elimination of Swift Fuels’ candidate fuel.
A more recent article, July 2019
A fuel under evaluation as a replacement for leaded avgas for the 170,000 aircraft in the piston-powered general aviation fleet will need “additional refinement,” and three new fuels have undergone preliminary testing in 2019, the FAA said in an update on its Piston Aviation Fuels Initiative.
According to the June 2019 program update, “PAFI's focus during the first 6 months of 2019 has included testing at the William J. Hughes Technical Center of an optimized Shell fuel and screening testing of 3 fuels not previously part of the PAFI program. The scope of PAFI has continued to evolve with the preliminary evaluation of 3 other fuels representing PAFI's commitment to research and evaluate all candidate unleaded fuels. Test results with the optimized Shell fuel were not successful with testing indicating additional refinements are required.”
AOPA's article appears to be an excerpt from this FAA article which has more details