It seems as though Protestants in general rejected the more ostentatious displays of faith and wealth that Catholics displayed in their churches, but what was their specific problem with the crucifix (the image of Jesus hanging on the cross, rather than simply the cross itself)?
The specific issue being addressed by Protestants preferring to display a cross rather than a crucifix is idolatry. The image of Jesus is not the only part of the issue as the accusation of idolatry is also made about the Catholic use of images of Mary and the Saints.
Partly this arises from the differing way in which Catholic and some Protestant churches summarise the Old Testament mosaic commandments given in Exodus 20: 2-17. There are fourteen imperative statements some of which are closely related by subject but which can be grouped in slightly different ways to form the well known Ten Commandments used for memorization and teaching. That is why there are different lists of the Ten Commandments.
The Catholic church does not teach the numbering of the Ten Commandments as dogma but generally teaches the grouping laid down by Augustine which follow the Jewish (and Lutheran) numbering. That is it considers Exodus 20.3 to 20.5 to be part of the same command, "I am the Lord your God, you shall not have other gods besides me. You shall not carve idols for yourselves in the shape of anything in the sky above or on the earth below, or in the waters beneath the earth; you shall not bow down before them or worship them." usually summarised as "I am the Lord your God, you shall have no other gods besides Me." to form the First Commandment.
Many Protestant churches use Exodus 20.3 as the First Commandment and Exodus 20.4 - 5 as the Second Commandment treating the injunction against graven images as separate rather than a reinforcement of the lines before it. They consider Catholic use of images to be idolatrous. Catholics also preach against idolatry but define it specifically as the worship of images.