Ok Kent Clarke, i'll bite.
Because it's an alternative tactic, one that Klopp and his band of ballbags couldn't cope with. Which is exactly the point in a game of football.
Mourinho is famed for NOT playing beautiful football, and despite being a manager in his own right for roundabout the same time as Klopp, has won more than Klopp will win in his lifetime. He didn't 'resort' to playing the long ball - he changed the personnel on the pitch to accommodate playing a more direct approach, eliminating Liverpool's midfield after Coutinho had come on and exposing their absolutely garbage defence.
The fact that United scored proved that the tactic worked, Klopp is cry-arsing about it shows him for the spoilt princess he is. Instead of bringing on a Brazilian midget, he ought to have bought another defensive midfielder on to shore his back line up. Mourinho countered his substitution by changing his tactics. It worked, job done.