If I were to put on my conspiratorial hat, I would guess the reason for stalling is that approving the G100UL fuel from General Aviation Modification, Inc. (GAMI) will divert profits from Shell and ExxonMobil, both of which provide significant funding for the two US political parties.

In fact, I would not at all be surprised to see approval delayed until such time as Shell or ExxonMobil can develop a competitor -- not an exact copy of GAMI's fuel, but something close enough to not violate the patent.