Graham,
Flexibility and speed of response are the main reasons. Ideally, an MPA should have a full 'sensor to shooter' capability for that instant reaction to a threatening fast and fleeting contact that needs to be attacked immediately. Turning and burning at low level ready to drop a weapon on a 'MAD' mark made this achievable as opposed to calling in the cavalry that may be orbiting above and could take a few minutes to line up on an attack run against a target that can't be seen.
Having said that, ASW helos and MPA regularly practiced working together with the helo directing the MPA as a weapons carrier. A helo in the dip was effectively a large active sonobuoy and the MPA could readily be restocked with torpedoes on return to base unlike the ship that may be at sea without a restock for some considerable time.