There are companies including mine, that are starting to support Macbooks instead of traditional PC hardware like Lenovo. The employees using macbooks are using either bootcamp or some emulator program to run Windows programs. If Apple moves to ARM they will be cutting off this inroad into corporations. Alot (and i mean over 50%) of mac users on consumer side also use one of these method for using windows programs.
Also, another reason is that not only does that Mac Os have to be converted over to ARM platform but also all their existing Mac OS software; we are talking about having every single branch of the company making this change...not easy.
I have heard that Apple may try keeping the current Intel Macbooks but also try to create an ARM version to test out the market. Considering how the MS Surface RT did, I don't think they will. People hate no backward-compatibility support.
My bet on Apple's next product will be a MacPad with Intel inside. A convertible with touch screen and slide out keyboard. It runs Haswell or some hybrid Haswell-ARM chip. Will run both ios and Mac OS.