Because it will lead to abuse and thus not try to measure political knowledge.
Because a reasonably accessible test can't really measure political knowledge. Even defining "politically knowledgeable" is hard. Do you need to watch Trump rally speeches to be politically knowledgeable? Do you need to know the three branches of government? Are we a democracy? Do we have free and fair elections? Can you be a single-issue voter, or do you need to prove you know all the other stuff?