The only thing worse than being stuck in the office on a summer day is being stuck in the sweltering heat for five and a half hours behind one slow foursome after another. OK, so maybe it's not worse than the office. But it's not good either. Golf has never been more popular than it is now. However, once people see how long it takes to play a round, that popularity might wane.
Some people attribute the slow play to that popularity.
The theory they're espousing is that more beginners are playing the game and it takes them longer to play. That may be part of the reason. But it's not the entire reason. Some people that have played forever take a long time to play. Beginners that play with these experienced slow golfers learn to play slow.