Prejudice is the answer, but there are many reasons.
Their labor is so cheap, no American worker can compete. Glass is glass, so where it is made only means who eats from the work done to it.
There are things to take into consideration also. Where taxes paid, many garage glassblowers do not pay taxes and take advantage of the American system. The end consumer only has so much money, and they may not want expensive glass, if they do not understand what make it cost more. This is your job to educate the consumer with why buy American glass.