Dan.....Winky made excellent points. Many Americans have a college basketball rooting interest, alumni connection, or other association with the players, coaches, or schools.
Almost everyone gets involved in some type of office pool, even those that don't bet.
Unlike the NHL, NBA, and MLB playoffs, this is a single elimination tournament. When you lose a game your season is over. This one and done bracket format has led to many upsets and surprising results.
This David vs Goliath factor along with the growing popularity of office pools and TV coverage has fueled the growth of this event.
Hope This Helps
Gary