A brief explanation why…
Finalized in 2013, 802.11ac increased 802.11n's theoretical maximum throughput from 600Mbps to 1.33Gbps and supports eight antennas versus four, but nonetheless offers around the same range as its preceding specification and arguably even less on paper. 802.11n defaults to a frequency of 2.4GHz whereas 802.11ac exclusively uses the 5GHz spectrum and the higher band doesn't travel as far.
The range offered by both specifications balances out to be about equal because there are less consumer electronics congesting the 5GHz spectrum and 802.11ac's beamforming technology can boost the signal strength where it detects a device instead of omni-directionally. Although you'll experience about the same maximum range from both specs, 802.11ac will deliver higher speeds in those distance spots.
If you're considering an upgrade to wireless AC, scan our list of the best routers you can buy right now.