Why don't Christians obey more of the Old Testament, and not just the parts on sodomy?

I'm confused. Now, all the good Christians I know are good at avoiding sodomy and condeming those who practice it because of Old Testament prohibitions, but what confuses me is how these same people don't take seriously the teachings on wearing clothing of different fabrics and eating shellfish? I thought that Jesus did not come to destroy the old covenant, but to affirm it. Why so selective?