There are at least two reasons for this.
First, it is difficult to know which political party is best because this is largely a subjective question. To answer the question, you have to have some way to define “best.” Generally, people tend to feel that a political party is good if it shares their opinions. Therefore, different people will have different opinions about which party is best and there will be no way to objectively determine whose opinion is right.
Second, it is difficult to know which party is best because it is hard to know whom to blame or credit for things that happen in a given country. This is particularly true in countries like the US where more than one party can control a part of government. For example, the US economy boomed in the late 1990s. The president at the time was Bill Clinton, a Democrat. Does this mean that the Democratic Party is better than the Republican Party? It is hard to tell since the Republicans controlled Congress for most of Clinton’s presidency. Even if the Democrats controlled the whole government, it would be hard to know if their policies caused the boom or if it was something beyond political control, such as the growth of computers and the internet.
For these reasons, it is very hard to say which political party is best.