Why are international students turning their back on American colleges?

Because many of them are fake and are afraid of being found out. That and they monopolize engineering programs in US colleges, which everyone has finally figured out.

https://www.marketwatch.com/story/the-number-of-international-students-coming-to-the-us-for-grad-school-declined-for-the-second-year-in-a-row-2019-02-07