The issue is: People Are Stupid(tm)
When AT&T rolled out U-verse here, their regular prices were competitive with Comcast's promo rates. So Comcast lowered their promo rates and increased their regular rates. Folks get drawn in by the lower initial rate, but don't follow-up on it.
Also, people don't try to get a better deal. My sister is moving later this month. She and her roommate would prefer to get U-verse, as it's cheaper; but it's not yet available where they will be. So, they were going to go with Cox, paying the going rate. I suggested that she contact Cox, asking them to match the U-verse rate; by simply calling, she got a six-month deal on their triple-play package that works out to less than they would have paid for U-verse.
These companies aren't so dense as to not take advantage of people's inability to save themselves money.