I'm not sure why Jesus changed the law about what is OK to eat. Why did he allow us to eat pork?
To understand why the law changed, you first need to understand why those laws were there in the first place. I would like you to read an article that I wrote called, "Uncleanness." It discusses the purpose of the laws of uncleanness.
The Law of Moses put a lot of emphasis on physical things to help the Israelites understand spiritual concepts. The spiritual concepts were there in the law, but the physical actions allowed people to better understand the things they could not see. Two thousand years of daily animal sacrifices allowed people to better understand the single sacrifice of Jesus on the cross. Having a holy tabernacle and temple allowed people to better understand the importance of holiness. The laws of uncleanness better-instructed people about the nature of sin. Now that the lessons have been learned, God moved us to real law that focuses on spiritual ideas. "So let no one judge you in food or in drink, or regarding a festival or a new moon or sabbaths, which are a shadow of things to come, but the substance is of Christ" (Colossians 2:16-17).
When God was teaching men about sin, it wasn't pork that made a person unclean, it was the breaking of God's law. "And He said to them, 'Are you so lacking in understanding also? Do you not understand that whatever goes into the man from outside cannot defile him, because it does not go into his heart, but into his stomach, and is eliminated?' (Thus He declared all foods clean.) And He was saying, 'That which proceeds out of the man, that is what defiles the man. For from within, out of the heart of men, proceed the evil thoughts, fornications, thefts, murders, adulteries, deeds of coveting and wickedness, as well as deceit, sensuality, envy, slander, pride and foolishness. All these evil things proceed from within and defile the man'" (Mark 7:18-23). Now that the lesson has been learned, we move to the application. "Therefore the Law has become our tutor to lead us to Christ, so that we may be justified by faith. But now that faith has come, we are no longer under a tutor" (Galatians 3:24-25).