Cost. USGI mags- the aluminum units are pretty good...... once you add MAGPUL parts to them. In reality, they work okay, and I have some. But the Pmags are just plain BETTER. I will give you a couple simple tests: Take a mag, and turn it on its' side, and lay it on the ground. Now, give it one good stomp. Not like you are trying to flatten it forevermore, but like if you were moving around real quickly, and dropped an empty and then accidentally stepped on it. Better than 60% of USGI mags will be dented in, and will not work properly. If you want to be fair about it, jump up and down on the Pmag like a chimpanzee going nuts at the zoo. The Pmag will win every time.
You can intentionally drop them, loaded or unloaded, on the feed side. The one positive is the USGI mag can be bent back. The other positive side is the Magpul bounces back all by itself, and you don't need to rework it. And, the feed lips on the Magpul will stay correct, whereas after you maim any metallic mag's feed lips, you will be tuning them.

USGI mags are good. Pmags are far superior to them.