People on mobile don't click as many ads. If you want to increase your mobile CPM you need to use "native" ads (part of the user experience, not a display/banner ad). Unfortunately that inventory isn't easily accessible to small, non targeted publishers. Mobile engagements are short and direct. People using browsers are more inclined to "browse" links including ads. Mobile users see ads as clutter. Display ads take up a much larger part of the display real estate on mobile vs. desktop, even responsive ads. A banner might take up 15 or 20% of an iPhone screen but only 4 or 5% of a normal browser window.