It is because of the following reason:
"He (Peter) saw heaven open and something like a large sheet being let down to earth by its four corners.
12It contained all kinds of four-footed animals and reptiles of the earth, as well as birds of the air.
13Then a voice spoke to him: “Get up, Peter, kill and eat!”
14“No, Lord!” Peter answered. “I have never eaten anything impure or unclean.”
15The voice spoke to him a second time: “Do not call anything impure that God has made clean.”
Now, the above is one of those scriptures which has a dual meaning. At face value, it shows that God deemed which animals were clean and unclean, else how could an animal at one time be unclean and then later be clean? The animals deemed unclean under the law were only unclean because God said they were. The figurative meaning was referring to Gentiles, who were previously unclean in relation to the Jews. But now that salvation had come through Jesus Christ, Gentiles were no longer unclean, but were made clean through Christ. We know this because of what Peter said below:
"As Peter talked with him (Cornelius), he went inside and found many people gathered together.
28He said to them, “You know how unlawful it is for a Jew to associate with a foreigner or visit him (unclean). But God has shown me that I should not call any man impure or unclean.
29So when I was invited, I came without objection. I ask, then, why have you sent for me?”