Why is football such a "pass happy" game

15-20 years ago, Football was a grind it out sport. The teams leading running back would get 25-30 carries a game. In today’s NFL, QB’s are throwing the ball 50-60 times a game. The Shot-Gun, Spread formations are more used then 15-20 years ago, when coaches like Bill Parcells, Bill Cowher used to love running the ball in the 4th quarter with a lead.