I studied mathematics in Mexico City as an undergrad and know plenty of people who tried to go to a math conference in the US but had their request for a visito

I studied mathematics in Mexico City as an undergrad and know plenty of people who tried to go to a math conference in the US but had their request for a visitor visa denied. (I've personally never been denied a US visa, and I think most of my friends have been able to go to the US whenever they want, but still plenty of people get denied.) The embassy doesn't give you a reason and there's nothing you can do about it. In the future the reason they don't tell you could easily be "It seemed fishy that he didn't have a twitter account" or "Last facebook post from a year ago? Must be a fake account". If you aren't told the reason it can be anything.