You pretty much answered your own question.
If you're playing in a metagame that isn't full of SP, then you're basically playing against a field that wrecks you. Machamp can't take out and loses to every other stage 2 deck, and Gengar is an autoloss to Flygon and anything that's prepared for it. The only reason Gechamp's popularity rose was because of the outcome of Japan's national championships. Though Japan was predominately SP in metagame, thus giving Gechamp(w/Relicanth) an easier field to play against.
So, if your area is deprived of SP's and is full of decent/good players, Gechamp is not the play.