WWE Raw has long been the older, more dominant brother. Take November of last year for example. WWE Raw averaged a 2.78 rating (h/t Wrestling Observer, via WrestleZone.com) while WWE SmackDown earned a 1.88 rating during that same period. This is a pattern that has largely stayed the same.
WWE isn't without options to even things out.
Some changes suggested here are minor and would require little effort. Others require new or altered deals with TV networks but would be worth the headaches in the end.