I haven't read much about the details, because personally I don't really care (we're having our own elections in 3 weeks, and that is much more relevant to me than anything in the US), but I think for most people it comes down to morality or ethics. Tax can be seen as a way of contributing to society, paying for what many consider to be essentials, such as law enforcement and justice systems. The fact that he has paid somebody to ensure he didn't pay tax can be seen as not caring for anything or anybody other than himself. Some may not want a leader that they may see as being that selfish, when they should be considering the needs of the entire nation.