By comparison, habilis, rudolfensis and ergaster not only lived in the same location at the same time, but also apparently evolved into distinct "species" in the same location and at around the same time. This is called "sympatric speciation": something other than geography was a barrier to interbreeding between these "species."
Or, rudolfensis and ergaster could both have evolved in small, geographically isolated regions outside the notice of palaentology, at least up till now, before expanding back into the rest of Africa. Is there anything I've missed that argues against this alternative?