It's because he clearly and admittedly renounced his citizenship for the purpose of not paying U.S. taxes. That bolded part is important. We don't need any conspiracy theories here, it's been U.S. policy for many years that if you renounce your citizenship for the purpose of not paying U.S. taxes you are not to be admitted back into the country. Eduardo Saverin (co-founder of Facebook) is likely in the same boat, since he also clearly renounced his citizenship to avoid taxes.

Of course, it could also have something to do with the fact that Ver was convicted of selling explosives on eBay, that's something I'd think most governments (not just the U.S.) wouldn't take too lightly.. but no, let's forget about any plausible explanations based on long-standing U.S. laws and policies and just assume it's a conspiracy against Bitcoin.