Probably because WP7/7.5 phones will not be upgraded to WP8. Why waste money marketing a device that will effectively "go away" this October when WP8 is released? Microsoft has already decided to invest heavily in WP so it makes much more sense to save their marketing dollars until their long term/future OS has been finished.

For those that are unaware, WP7 and WP8 are built off of different kernels.