Quite simply direct-lit is cheaper to make than edge-lit. Direct-lit basically is the same design as old CCFL-lit but replaces the CCFL with LEDs. The reason edge-lit is more expensive is because they need to add lightguides, etc to distribute the light across the screen from the edge of the screen. In direct-lit, the led's are instead distributed behind the screen at a distance so they naturally distribute their light across the panel. In addition, this positioning of the LED's requires fewer LEDs than edge lit. Direct-lit should not be confused with the old full-array LED which, while still placing the LED's directly behind the screen, is done at a much closer distance using many more LED's. Full-array sets are quite expensive to make.
Direct-lit Tv's are fat like old CCFL lit TV's. The consumer demand is for thin TV's so besides being more expensive to make, manufacturers are positioning the thinness factor as a premium feature with edge-lit TV's. The very top end TV's for most manufacturers utlize the old full-array set-up with local dimming, which tend to provide the best PQ out of all LCD formats.