First and foremost, because Jesus fulfilled the law, but did not abolish it. I'm not on pins and needles every stinking moment ensuring I dot my t's and cross my eyes... or something like that. I try to do the best I can to mirror Jesus, and also know I will fail.
Second, within the Pentateuch, there are 3 types of laws: ceremonial, civil, moral. Civil is early Israel, and don't apply. This would such as the slave laws. Ceremonial is either priestly or cleanliness. Those don't apply for the most part, but some do. That includes shellfish. Finally, most of the moral laws still apply, that didn't get excluded due to civil laws.