[doug*platypus]

As the Doomed Khazad accurately points out, the losses of our poor, motherless heroes are almost without exception never used to generate sympathy. Modern movies, and of course people in general overplay the loss of a parent, but this does not happen in Tolkien's writing.

Tolkien's written world is strongly patriarchal. I think that in most cases, once our hero has been born, their mother is simply no longer required. A hero who has experienced some loss in the past has more of a story to tell, even if they are not hung up on the fact. But a hero without either parent is missing out on something, deficient. They are labelled as an orphan, and must continue through life on a different level to those still with at least one parent. So orphans are as rare as characters with both parents, because they are special. A character with 'only' a mother instead of a father in Tolkien's patriarchal written world, might as well be an orphan, and so these characters are rare as well.

Also, since Tolkien was without his own mother, perhaps he did not feel able to write about the sometimes tricky relationship between sons and mothers. Maybe he avoided that difficulty, as it has been suggested that some writers do. Whatever the reasons, it is very interesting to note the number of "matricides". Thanks again!