While I was not able to find anything explaining why these particular women figured in the count as opposed to others, I did see an interesting insight into the pesukim in consideration of that fact. Tzeror HaMor and Emes L'Yaakov both point out a discrepency in the per-wife tallies found in the pesukim. The numbers for Rachel ("arba'ah asar") and Bilhah ("shiv'ah") are of the masculin form. The numbers for Leah ("sheloshim veshalosh") and Zilpah ("sheish esreih") are feminine. They both explain that Leah and Zilpah both had women counted among their offspring - Dinah from Leah and Serach from Zilpah. Therefore their numbers are delivered in feminine. Rachel and Bilhah had no feminine offspring counted and thus their numbers are in masculine.
One might wonder why this is so, considering that the generic plural is usually masculine by default. However, Emes L'Yaakov points out that the word "nefesh" which the number is qualifying is feminine. So the default gender of the number for "nefesh" should be feminine. Rachel and Bilhah were the exceptions.