Same reason why there are 5 BCS bowls. College Football is a step away from communism in that almost everyone walks away a winner.
More seriously, there is no "official" MVP award for college. Over the years, random organizations have decided it'd be cool to hand out awards. The first and most prestigious was the Heisman, awarded by the Downtown Athletic Club in New York. The Downtown Athletic Club was just that- a club of wealthy guys who liked football. Over the years, the award became more and more prestigious and became a bigger and bigger deal. Other entities have jumped in with awards of their own, awards for the best interior lineman (as opposed to the best exterior lineman?), awards for the best TE, the best punter, the best DB, the best defensive player, and the MVP. The Walter Camp guys decided they didn't like the Heisman guys hogging all the glory, so they started handing out their own MVP award. The Manning family decided that people weren't sick of them enough yet, so they started handing out their own MVP trophy, too- although they wanted to be the only award presented after the bowls were played.
If we really wanted, we could form some sort of FBGs coalition and create our own MVP award that we awarded every season. We could commission a goofy-looking trophy, buy all the finalists round-trip airfare to Atlanta, treat them to a night at Six Flags, and host a presentation ceremony on local access cable. Our award would really be no different than any of the others except in terms of the esteem in which it'd be held, and also in the fact that defensive players and linemen might actually stand a chance of winning our trophy.