Besides what's been mentioned already (John's point about providing power is my first go-to for this question), keep in mind that since it operates within consumer spectrums, WiFi is prone to all manner of interference from other consumer devices, which can greatly affect reliability. This may be improving with the new 802.11ac standards (which still uses 2.4GHz, but simultaneously with 5.8GHz), but anything operating in 2.4GHz is still prone to issues - as as extreme example, I've had a system where a laptop would lose its WiFi connection every time its 2.4GHz wireless mouse was used.
You could get into more proprietary, unfettered spectrums, but those will get really spendy... AND you'll STILL have to power the cameras somehow.