They have a lot more control over the mobile processors. The laptop processors have to maintain compatibility with all the software that people run on their laptops; effectively, that means they're stuck with other people's processors.
I think the real reason is some combination of (a) ARM isn't competitive (or only recently became competitive) at the high power/high performance point that Intel CPUs excel at; (b) the Mac line generates so much less revenue and operates at so much smaller of a scale than iOS devices that it isn't worth developing CPUs in-house for them, not to mention the fixed costs associated with undergoing a transition.