Any quorum of college coaches put together to answer that question would give you three quick reasons:
1) Outdated offenses. Football is a dynamic game, invigorated by continuing change. Most college coaches phased out the so-called "pro set" and its one-and two-back running capabilities years ago in favor of the more versatile wishbone, veer, power (and option) I, etc.—and combinations thereof. The better college offenses today are three-and four-back offenses. (A "back" in this case means one who runs the ball; a quarterback used exclusively as a passer is therefore not included, nor is a "flankerback" used only as a wide receiver.)
2) Coaches fearful of change and/or of getting valuable quarterbacks hurt. Tactically, pro coaches are scared to death of option offenses. The fear's tranmuted into dogma: Option offenses imperil the quarterback. Lay off. Traditionally, pro coaches are offensive conservatives. Having much to lose (their high-paying jobs mainly), they tend to coach along duplicate lines and resist change. They draft talent and try to make it fit style, instead of vice versa. Innovations in offensive play invariably begin with the colleges, and usually to accommodate a player (or players) with unique talents. In other words, college coaches adapt better.
3) Offensive linemen who aren't schooled—or, rather, have been systematically unschooled—in effective rush-blocking. An offensive lineman who spends the great majority of his time pass-blocking soon has his ability to block for the run dulled. The techniques are as different as the acts of driving and putting a golf ball. Pass blocking is basically a defensive posture, a flat-footed "shielding" technique; rush blocking is essentially an act of aggression, requiring an explosive charge (what coaches call "firing out").
When Bud Wilkinson temporarily took leave of the television booth and his various business interests (and some say his senses) to return to coaching in a stab at revving up the moribund St. Louis Cardinals in 1978, he found what he had suspected of NFL running attacks to be true. That they were hamstrung by their own ways and means. Wilkinson's University of Oklahoma teams won national championships and went on long winning streaks; he was known for the thunderbolt strikes of his running attacks, much as Bear Bryant, Barry Switzer, Tom Osborne and Emory Bellard are known for theirs today.
Wilkinson is unconstrained to say what a lot of his fellow coaches hesitate to say (for fear of ruffling the brotherhood): that a two-back offense is a limited rushing tool unless you're blessed with overwhelming physical advantages, especially at the line of scrimmage. Without at least a three-back offense, says Wilkinson, "you have a terrible time getting any meaningful misdirection." Misdirection plays—the counters, the inside and outside reverses, the bootlegs that bring a play back away from the flow of the blocking and into the soft underbelly of the pursuit—are the bane of heavy linemen committed to a directional charge. The most deceptive of running plays, they create advantages in blocking angles and are designed to spring backs for long gains.
The real crux of the problem, however, is pro coaches' prejudice against any running play that involves (as the "third back") the quarterback—the bootlegs and, most especially, the options. The very best coaches of recent times—coaches like Wilkinson, Bryant, Darrell Royal and John McKay—agree that the toughest play to defense in football is the option play. A good option offense, from the split T of the 1950s to its modern descendants, the triple-option wishbone and veer (and Bellard's "wingbone" at Mississippi State), inspires the imagination of offensive coordinators. They love it when defenders have to tackle two or three backs on every play to make sure they get the right one.
Bear Bryant found something that ought to interest the pros about the triple-option wishbone—that the defense had to commit so many people to defend the first two options, it always left the quarterback with one-on-one coverage of his pass receivers. "It was automatic—we got it every time," Bryant says. Getting one-on-one coverage for their receivers is something pro coaches scheme and slave over from the first day of practice to the last.
Pro coaches are united in their conviction that an injured quarterback is the price you inevitably must pay for an option offense. Is it a valid fear? Probably. And presumably. And maybe. But maybe not. Even the more successful college coaches who defected to the pros (McKay is the best example) certainly have been slow to risk challenging the dogma. Wilkinson himself didn't do it, but says he was toying with the idea of incorporating option plays into his St. Louis offense when his stay there was "aborted" in 1979. He would do it today, he says, but admits it might be foolish to try without two good running quarterbacks. By the same token, more college teams than ever use option offenses, and good, tough (and sometimes very large) running quarterbacks are much more plentiful.
Nor is Wilkinson convinced that option plays are as dangerous as some of his colleagues think. "I never saw a quarterback get hurt [badly] running an option," he says. "I've seen a bunch of 'em get hurt dropping back to pass." He means that when a quarterback strings a play out to the sidelines, as he will do on option plays, the chances of his getting hit much more than a glancing blow are not nearly as great, say, as when he is at the vortex of a disintegrating pocket, taking direct hits from three or four 260-pound linemen.