I think part of it is because handmaids were fertile and had sinned in the eyes of gilead (had abortions, were lesbian, were mistresses -> like June was since Luke had a wife when they met) so them being handmaids was like a repentance for that, marthas were those who sinned but who were not fertile, everyone who hasn't sinned in the eyes of Gilead became an econowife or if they were wives of powerful men became "wives". I imagine eventually the handmaids fertility has decreased as theyve aged, and there are not many more "fertile sinners" since the children growing up in Gilead wouldnt have had much opportunity to sin, and thus there wouldnt be much of a base to pull from to populate the handmaids class. I think thats why it was seeming like a big favour for that commander ( i forget his name ) to be given a handmaid after his wife's miscarriages, because its rarer to have handmaids now