Why were protestant ideas slow to gain popularity in southern europe during the reformation?

Because of the influence of Roman catholic

Explanation:

Many of the Protestant ideas actually contradict the practices that carried out by Roman Catholics. (For example, the protestant does not believe in church practice of selling indulgences to abolish believers' sin.)

This made the spread of Protestantism faced heavy opposition from Roman Catholics who already had a strong base in this territory. This is why the ideas were really slow in gaining popularity.