There was problem with old one. With STANAG dimension magazine inserted Colt magwell allowed too much dust, sand, etc to enter into upper receiver, even with dust cover closed. HK could not control magazine dimensions, so they solved problem by making tighter and longer magwell, that reduced dust penetration into receiver.
Some time later other company designed magazine, that is not within STANAG dimensions, but is made to fill up and seal Colt magwell and prevent dust penetration. But when HK were designing 416 nobody even dreamed about P-mag.
USMC did not specify that IAR must use non mil spec magazines like P-mag, so there was no point for HK in reducing system reliability by going back to old magwell.

