According to
link, the legend is that "Maternal figures were typically absent in Walt Disney's animated films because he felt responsible for his own mother's death."
However, Snopes points out that the lack of mothers in stories is not unique to Disney films. Several fairytales include the Wicked Stepmother, who usurped the absent mother's position. But wewe can also find it in literature. Orphan characters (motherless and/or fatherless) appear throughout history. Consider David Copperfield, Frank L. Baum's Dorothy, au even Harry Potter.
It's quite a common convention that existed before and after Disney's time.