There are about 9.4 million families of two or more people. Of those, about 672,000 are single male earners, and they have a median income of about $40,000. Income splitting won't do anything for them either, because they're already in the lower tax bracket. There is nowhere further for them to go.
It's not only that it doesn't do anything for single-parent families, it's also that it doesn't do an awful lot for the families who may only have a single earner but two parents and are the ones who are actually struggling.