This is a question that we'll never know the right answer to, but we sure could use any answer! (Like the question Which group of Muslims are the closest to the Way of the Prophet (S)?) If we only knew...life would be so much more simple!
I think it goes back to Adam & Eve. It took only one woman to provide companionship to Adam. Now, however, men have multiple women in their lives, all in a way, competing for a very similar role. The mother gets the first shot, in raising her son and being there for him. But later a wife, sister, a girlfriend, etc. enter the scene and her role is diminished.
So mothers I think, do whatever they can to limit the influence of other women in the lives of their sons. I think they do this because the feel their connection is genuinely threatened. Because...men dont requrie much at all to be made happy by a woman. And a wife can give it all. And i mean all. Which the mother and sisters can never give. Which is why both mothers and sisters can be a source of so much stress to both the wife and the husband, because the wife now holds the winning hand...unless she screws it up! :)