The Old Testament does not give us an explicit reason for this. But here are possible reasons for it:
First, the animals that were labelled as unclean back then may have contained harmful micro organisms at that time. Hence, the prohibition. Hygienic and health issues.
Second, it was to symbolically inculcate in Israel the spiritual lesson that, in this fallen world, there is such a thing as being clean and unclean and the need to be spiritually cleansed. The symbol was a sign that points to the spiritual.
Third, it was to separate Israel from the surrounding nations. The unclean food that were prohibited were the regular diet of pagan nations. Fellowship is usually illustrated by two people sharing the same meal together. The categorization that this food is unclean while this one is clean was to create a separation between Israel and the pagans.
