Here is the big reason...
Cingular was well regarded throughout the telecommunications industry, as it was a grassroots phone company that made it big. Because of this, they were very helpful to other service providers when it came time to negotiate contracts for sharing access to owned cellular towers.
When AT&T bought Cingular, those licensing deals went out the window because AT&T doesn't feel that they need other carriers in these markets. Where needed, they can lease space on third-party towers and build new ones where necissary.
In turn, they could restrict access to "their" towers to claim better coverage in areas than their competitors.