the talent pool for those weight classes is super small in mma
because countries that have a lot of small people and a fighting a culture are not really into mma that much(other then japan)...
mexico and thailand are good examples
both produce tough fighters in different but related sports
barely anyone in those countries cares much about mma...
even in boxing the talent pool at smaller weight classes under 135 pounds is VERY small compared to say 160 pound weight class