There is a point system used to grade guns being considered for import.
Larger size, larger caliber, and things such as target triggers, thumb rests, adjustable sights, etc. (sporting features) earn more points.
If the gun doesn't earn enough points it's not eligible for import. The .380 caliber earns very few points which means that .380 and smaller caliber pistols end up with a point deficit that's hard to get out of. The Glock .380 doesn't have enough other "sporting features" to make up for the loss due to being chambered in .380. Glock could make them in the U.S.--that would be perfectly legal--but importing them is not.
My guess is that Glock doesn't figure the demand (profits) would be high enough to warrant the hassle of modifying them to garner the proper number of points.