Early wireless AC card were junk. Many of them ran wireless N faster. This is when the Surface Pro 2 was in development.
So it was a natural move for Microsoft to put a stronger and better wireless N, than AC, which would simply result as being an cons in reviews due to it's poor performance. (at least based on the reviews I read about). Remember I am talking about the early ones, when the Surface gen 2 was in development, not the ones available now, which are better.
In addition, Microsoft was limited in using Bluetooth + Wireless card 2-in-1 solution due to the size of the device. More work needs to be done to minimize the internal circuitry to allow 2 separate cards, and bluetooth own antenna. That is if they choose or even can do that, and not simply opt for a 2-in-1 card with better, proper, wireless AC support (let alone exists) on the next generation of Surface devices.
Finally, wireless AC is in draft. Early draft. I remember quite clearly when wireless N was out in its early draft version. When a new draft came out, everyone that had the early wireless N router and cards where out of luck, they had to change everything. Imagine having the same story with your Surface Pro 2? In other words, wireless AC would work at home with your old router, but if you go outside to coffee shop, school, or work space, you are stuck to wireless N, as they use newer draft version of AC.