Why were purity regulations stricter for priests than for other Israelites?

There were stricter purity regulations for priests in the ANE than there were for other Israelites. This was because the priests were going to be going to a holier place than the other Israelites (the inside of the tabernacle). They needed to be cleaner, or more pure, in order to go closer to God. They found things impure if they were mixed, incomplete, or disorderly.
If I were to go to a king’s palace, I would most likely wear a dress, or at least very formal clothes; I would not wear my pajamas. This would be mixing two different cultures and times. It seems obvious that I would not do that. This is the same for the priests. They needed to be clean, and be appropriate, in order to be closer to God. Since God is the most holy, God deserves to have perfection near God. It wasn’t appropriate to come to God without being as pure as possible, and not just any Israelite could go into the tabernacle, because they were less pure than the priests.
God was separate from the Israelites, but yet still in their presence. God just deserved so much reverence that all the respect that the Israelites could show God, they did. By making sure that they were as pure as possible, and going through all the rituals of cleansing themselves, the Israelites were showing that they understood that God was the holiest and deserved honor. All the rituals were a constant reminder of God’s holiness. So since the priests were the ones that were going closest to God, then they had to make sure they were more pure than others that would not be as close to God.