Nationals of mainland China, Mexico, and India made up about 43% of all nonimmigrant visas issued by the State Department in FY 2018. That includes for business, tourism, temporary work, and schooling. Brazil, Argentina, the Philippines, Colombia, Israel, Ecuador, and Nigeria round out the top ten.
Note: Citizens of many countries (including Canada, most EU states, Australia and South Korea) can enter the United States for up to 90 days for tourism or business purposes through the Visa Waiver Program.
The Migration Policy Institute as a long list of possible explanations for the decline:
The recent decline may be explained in part by the Trump administration’s immigration priorities: a series of executive orders and policies have tightened admission and visa issuance criteria. Among them: additional vetting procedures for certain nonimmigrants seeking to obtain or extend their visa; restricted definitions of certain specialty occupations for temporary workers; a ban on visas for nationals of seven predominantly Muslim-majority countries deemed to represent security threats (often referred to as the “travel ban”); and visa sanctions against countries that fail or refuse to facilitate the return of their nationals ordered deported from the United States. The decline may also owe to a perception that the United States has become a less welcoming place, amid rapid policy change and at-times harsh rhetoric, including by leading government officials, against immigrants.
These changes have also been cited as one of the reasons for the decline in new international student enrollment at U.S. colleges and universities, which dropped for the third year in a row. Other factors include diminished government funding by countries such as Saudi Arabia and Brazil for their citizens to study abroad; changing policies in Australia, Canada, Japan, and China, among others, to increase recruitment of international students; and stepped-up efforts by traditional sending countries such as China, India, and Malaysia to offer higher-quality education at home.