Biblically an answer as to why the Kocher dietary rules do not apply in the New Covenant is found in Acts 10: 9-16  where Peter,  by a voice from heaven told  to eat even “unclean” animals because ‘What God has made clean, you must not call profane’ (Acts 10,16).

Then more ‘theologically’, the Old Covenant has been definitely  fulfilled by Christ who has established the New Covenant.  St Paul says, “We ourselves are Jews by birth and not Gentile sinners; yet we know that a person is not justified by the works of the law but through faith in Jesus Christ” (Gal 2:15-16).