The Lord Jesus has made it clear to me that God considers all foods fit to eat. But if you think some foods are unfit to eat, then for you they are not fit.
Not everyone knows these things. In fact, many people have grown up with the belief that idols have life in them. So when they eat meat offered to idols, they are bothered by a weak conscience.
Everything is pure for someone whose heart is pure. But nothing is pure for an unbeliever with a dirty mind. That person's mind and conscience are destroyed.