When you buy a phone with AT&T, roughly half of the monthly cost goes towards paying back the ~$200-400 loan they gave you to subsidize the phone (it works out to some absurd interest rate, roughly 15-30% depending on what assumptions you make). Of course, they charge this even if you don't get your phone subsidized through them so in that case it ends up just being really expensive.
With Straight Talk, they don't subsidize the cost of the phone, so they charge less on a monthly basis.