there might nog even be a 'real' reason. This happens a lot more than you may think. Visa waivers can be arbitrarily rejected (it's in the small print), if you

there might nog even be a 'real' reason. This happens a lot more than you may think. Visa waivers can be arbitrarily rejected (it's in the small print), if you encounter an official having a bad day, you may be out of luck. If your visa waiver is rejected, you are suddenly illegally accessing the country (you get put in the next flight back by armed officers; if there is no flight back, you get to wait in a federal prison until there is). Once back, you can never apply for a waiver again, so it's up to getting a real visa. The process is not made for that at all, since they except anyone from the waiver countries to just get a waiver. That results in long processes, many, many forms, long interviews, and an arbitrary time period, after which you may or may not get the visa (again, seemingly up to the arbitration of someone needing to not have a bad day).