Eddie,
The main reason is probably the gov't CAFE regs. In addition to seeking more HP from their engines, Ford, like everyone else was/is looking to squeeze more MPG out of them without losing power to meet the ever-rising CAFE regs.
The pushrod small-block Chevy/Ford engines were designed in a time when fuel economy was hardly a factor at all (the '50's for the Chevy, and early '60's for the Ford). Once the CAFE regs came about in the '70's, the Detroit automakers tried to keep up with them using existing engines & technology as long as they possibly could to keep costs down. By the mid-'90's, I think that Ford/Chevy had squeezed just about all the MPG of them possible without giving up HP. That's why the 302 was replaced in cars with the modular 281 V8, and eventually in trucks with the 281 and 331 V8's. The modular engines had the potential for significantly higher MPG and HP at the same time. And I think the same will be true of the direct-injection and EcoBoost engines. They may eventually completely replace the current modular engines.
If Ford can achieve 305HP/31MPG out of the 2011 3.7LV6 (direct injection) Mustang and 365HP/350 lb-ft of TQ out of the 2010 Taurus SHO EcoBoost 3.5L V6, just imagine what kind of HP/TQ/MPG Ford could attain from an EcoBoost V8!:banana: