First, tax collectors were seen as collaborators with the Roman occupiers. Judea had been under Roman rule for over a century by the time of Jesus, and the Jewish people were resentful of the Roman oppression. Tax collectors were seen as tools of the Roman government, and were therefore associated with the hated foreign regime.
Second, tax collectors were often corrupt. They would often demand more money than was owed, and would keep the extra for themselves. This made them even more unpopular with the Jewish people, who were already struggling financially under the Roman occupation.
Third, tax collectors were often seen as greedy and immoral. They were accused of being dishonest, untrustworthy, and willing to do anything for money. This made them an easy target for criticism from religious leaders, who wanted to warn their followers about the dangers of greed and materialism.
As a result of these factors, tax collectors were generally despised by the Jewish community in the time of Jesus. They were seen as collaborators, corrupt, and immoral, and were therefore avoided and shunned by the Jewish people.