Basically 3 things. Engineering, Materials & Warranty.
Engineering: BMW built the car and has all the exact dimensions. The OEM bumper will be designed to their exacting standards. Moreover, BMW probably spends the time and money to run the thing through a wind tunnel after it was installed by them and painted with their factory paint to ensure not necessarily that it's better than stock, but that at the very least, it's not worse.
Materials: BMW uses their OEM quality materials.
Warranty: All BMW parts have a 2 year warranty, IIRC.
Not at all saying the aftermarket is crap, just doesn't go through all the same crap as OEM does.