It has to do with the number of models they have to build I think. A limitation they covered up with in-universe contrivances (Salarian females never serve in the military, Asari are mono-gendered, Krogan women are valuable and closely guarded etc.)
I think female Turians showed up in the recent comic (IIRC they're more or less identical to males save for very short crests that hug the skull rather than sweeping back.) A female Krogan also appeared in the E3 gameplay demo. Of course she was wearing a burqa type garment so they're still trying to sidestep it.
Personally I can't picture female Elcor looking (or indeed sounding) any different than the males and as for the Volus; how would you tell the difference when they're always in envirosuits?