Why is 802.11ac 5 GHz only?

The most obvious place that speed comes from in 802.11ac is wider channels. Just as 802.11n provided a big speed boost by doubling channel width from 20 MHz to 40 MHz, 802.11ac uses 80 MHz and 160 MHz channels. With only 83 MHz of spectrum available at 2.4 GHz, there is only one channel available that too you are at the mercy all other devices keeping out of your way.

802.11ac’s second big feature to boost speed is 256-QAM, which requires clean spectrum to keep your error vectors small.

Reference:
http://blogs.aerohive.com/blog/the-wi-fi-security-blog/why-is-80211ac-5-ghz-only