Because there is little norm for us to use. In math, you have mathematical rules. In computers, you have standards. In science, you have mainstream knowledge. But as for politics, we don't have a whole lot of common ground that encompasses everything. There are groups that form around a common belief (pro-life vs pro-choice), but there are few absolutes to compare one to the other other than some middle, which isn't guaranteed. So we have to see everything compared to us, and whatever guides we have to sanity. Also, we have to judge politicians on our experiences and group expectations, except the politicians are more public. So they may be just as we are, but we see more of the bad because the dirty laundry is just floating in the breeze.
Then again, power attracts the corrupt.