I think it really comes down to ticket demand. For Florida vs Colorado, the away team only gets 3,000 tickets which is OK because due to travel distance that's probably a fair amount.
3,000 wouldn't be enough for UF vs UCF because it's a shorter trip and demand would be higher.
As an example, UF/FSU currently allots 6,000 tickets to the visiting team.
It puts both schools at a point where compromise doesn't make sense. UCF doesn't have 6,000 tickets to offer Florida for a game at Spectrum, much less a similar deal to USF with at 12,000 ticket allotment for two 6,000 allotments in Gainesville.
For Miami/FIU, they solved a similar problem by moving the game from FIU's stadium to Marlins Park. Still small, but 50% more tickets than playing at FIU and was really the only option as the other choice was for FIU to move their home game to Hard Rock (which is what Miami requested).
We know that the Citrus Bowl is off the table for UCF for a variety of reasons.