In order to understand why people are predjudiced against Heuristics, we must first understand what is meant by Proof.
Proof is a concept first thought up in the Age of Reason by Greek Philosophers who were attempting to teach students how to think. In order to do this, they developed a course based on a game called syllogisms. The idea was to build up a set of statements that were not assailable by other students. This set of statements called a "Proof" would then be challenged in a special type of argument called debate.
To Achieve this goal, a set of rules called Logic was developed that would retain the truth of the statements Students that understood logic, were more successful at making their statements unassailable, and thus would win the debate.
This ability to win debates, was thought to be quite valuable by leaders of countries. As such, the rules of logic, have been propagated as a sort of ideal of good thinking technique ever since.
While Logic is valuable especially when you want to argue about things and not be assailed by other peoples thoughts, and Mathematics, turning everything into numbers, is also important these are not the only valid ways of thinking. But because the Philosophers have sort of given them a patina of idealistic value, we often discount other types of thinking in comparison.
Heuristics are just one of those things we discount.