Unlimited internet cost has a much larger exponential factor based on area than basic capped service.
Wireline service like google is offering is much much more forgiving on bandiwiidth. High usage takes much less of a toll on equipment, and worst case scenario, it's self limiting, (too many people on at once, the internet slow proportionately). The hardest thing about wireline is the initial cost of wiring which google has the customer pay for.
Wireless service is much much more difficult. Towers take quite of bit of maintenance. The equipment is much more sensitive/(imagine a radio tower trying to to to several hundred or thousand connection simultaneous, without bleeding into each other). Each cell phone is a unique frequency. The new LTE allows for a little bit more fine tuning of the frequency.
FCC only allows a certain range that each company can use.
Basically, unlike the wires, the towers are much more expensive to put up, less reliable, takes more maintenance, and provides less bandwidth/speed. If too many people use it for to much, people start dropping calls ect
Some companies like Sprint, and to a lesser extent T-mobile reinvest most of their money into trying to keep up their infastructure in a small area. Small areas mean less maintenance. They also are a little more conservative in their selection of phones.
AT&T, for better or worse, has a broader coverage area, including in areas that reciever heavy weathering, (require more maintenance). AT&T also spends more money to make sure we have contracts on a very large variety of phones.
Basically, what it boils down to, AT&T doesnt have the infrastructure/budget to provide unlimited or something similar in the broader coverage area, compared to some companies.
Verizon has same difficulty, because they have a simarly large coverage. it's why they no longer offer unlimited either.