One inhbitor is patent issues. Shimano and Campy have patented just about every brifter configuration they could think of and even SRAM had to scramble to come up with their "Double Tap" mechanism that avoided patent infringement. Some of the early patents may have expired but there are still problems getting around current ones.

SRAM is indeed US based but almost nothing they make or sell is made here.

There have been attempts to create an all-US made group (Paul tried) but the products were either extremely costly or didn't perform all that well or both.