I asked myself the same question during the Minutes to Midnight era. Before the album or What I've Done was released, this picture (http://madamehunter.com/wp-content/uploads/linkinpark.jpg) was being used in the Linkin Park newsletter. Mike was at the front and Chester in the back, so I thought MtM would feature Mike more.
I don't disagree, and I admit that a frontman has to (re)present the band, but appareance is also something. He is/was the frontman of the band in the previous albums and that doesn't mean that he can't be it anymore now Mike is doing more.