Black drives the bishop back, so as to gain the ability to play ...g5 and break the pin at a later time. 5. Bxf6 Bxc3 6. bxc3 Qxf6 gives up the bishop pair and lessens White's ability to defend the weak c pawns, especially once they become fixed in place with ...c5, so the only good move is 5. Bh4.

Wikipedia has related information at Nimzo-Indian Defence.