The dietary restrictions did not exist before the Law was given to Moses. Remember, dietary and other cultural laws were set in place for one main purpose, to make the people of Israel very "peculiar" to other nations to the point where intermarrying would be very difficult.
God intended for all the nations to be blessed through Abraham's line. This blessing was to come in the form of the Messiah. In order for this to happen, Abraham's descendants had to remain a distinct people. This was easy enough up until the time that they were leaving Egypt, but once entering the land of Cannan, God knew that they would have problems with ignoring his admonitions not to intermarry with whoever was left after the conquest.
These cultural and dietary laws would serve as a wall to prevent the dissolution of the Israelite people through intermarriages as most members of other nations would not want to adopt Israelite customs and those who followed God's Torah from the heart certainly wouldn't be marrying outside of the 12 Tribes.
The plan worked. Jesus did come as a descendant of the Tribe of Judah as prophesied by Jacob at the end of his life and Moses later. Once God's plan for salvation for the Jew first and then the Gentile was completed, the world soon entered into what Paul refers to as the "time of the gentiles" and the dietary restrictions were done away with as they served no purpose anymore.