The single really prohibitive reason was noise.
The unducted fan engine is like the name says a fan engine with no duct. Nowadays the duct is an extremely important thing for noise reduction.
That doesn't mean that the duct is the only thing which infuences noise. Shape of the fan blade is also an important issue.
But I think that it proved impossible to create a high speed unducted fan engine which could meet present day noise criteria.
An ordinary turboprop engine is of course also sort of unducted fan. But it isn't feasible for speeds near the Mach 0.8 region. It keeps noise down by having a massive "fan" diameter and consequently a lower, subsonic fan tip speed.
The problem is really the constant supersonic boom of the fan tips.
Always keep your number of landings equal to your number of take-offs