RealWorldTech explains why Apple is unlikely to adopt ARM for its MacBook lineup, you can read it over here.
Rumors aside, Apple will not switch their laptops to ARM any time soon. In the past, Apple successfully made two platform transitions, when their hardware partners had no performance competitive designs.
However, x86 has no problem keeping up with the market in terms of performance or efficiency, and there are too many technical and business challenges for an ARM migration. Some of the key issues include:
ARM microprocessors are designed for lower performance and unlikely to match x86 performance in the next few years
High-end ARM designs may not have a power efficiency advantage over x86
Emulating x86 on ARM is necessary for compatibility and reduces performance and efficiency even further
Thunderbolt I/O is probably not available on ARM
No second source ARM vendors for key components such as the graphics processor
Apple is fundamentally a systems rather than a semiconductor company
Intel and AMD already do an excellent job competing to meet Apple’s needs
Moreover, Apple's chip designers are better suited to enhancing the iPhone and iPad to fend off commodity Android systems, rather than worrying about laptops. Based on our analysis, we conclude that Apple will stay with x86 for now and we examine how they might consider using ARM in future MacBooks.