The new WiFi standard (802.11ac) is gaining more and more traction. The most recent MacBook Air updates have it and Apple is even selling 802.11ac AirPort Extreme base stations. Why doesn't the new iPhone 5s have it? There simply wasn't a high enough ROI for Apple on it yet. They have a history of leaving out new communications technology in the iPhone.
- Original iPhone: No 3G
- iPhone 4: No LTE
- IPhone 4s: No 5 Ghz WiFi
What drives Apple to leave these hardware features out?
1. Battery life
Battery life is always a big factor when adding new technology. Apple has been continually advancing battery technology in iOS devices, but it usually ends up being a breakeven gain because of new technology that is power hungry (retina, LTE, etc). It's possible that 802.11ac chips are using a tad more battery than the mature 802.11n chips
This not only includes the cost of the chip, but also cost the of supply chain changes. The iPhone 5c and the iPhone 5s have the same WiFi chip, so it's another part that Apple can buy in greater volume and get a greater discount on. There is also the question if the chip vendors could meet Apple's quantity demands with a new technology.
With any new technology, the size of the chips shrink over time. Apple has packed the iPhone internals and not an inch of space is wasted. If the 802.11ac chip is any bigger, Apple would have to find something else to shrink.
I think Apple can afford to play the slow hand here. This is similar to 2009 and the iPhone 3GS only having 802.11g support despite 802.11n being ratified. Apple knows it can wait a year and build it into the design of the iPhone 6. The chips will come down in cost, get smaller, and use less battery.
The bottom line is that Apple generally doesn't introduce new communications technology into its 'tock' releases. That is generally something left to the 'tick' releases.
Update: As Stephen Hackett said, this is some 'tock' release since it's 64-bit and includes Touch ID. I consider it a tock release since it kept the same form factor as the iPhone 5 and it wasn't called the iPhone 6.