Where/when/how did ham become the traditional meal for Easter?
I think tradition is probably more regional / cultural than anything else.
Having lived in Hawaii 17 years and being around a lot of Orientals, my mother sometimes served "Sukiyaki" (Japanese stew) or "Tempura" (battered fried meats - shrimp, fish / vegetables - onions, carrots, green beans, etc.). Here locally in Eastern New Mexico, we'll probably have "Carne Asada" (Mexican beef steak) and enchiladas at church. At local restaurants we'd have roast beef or prime rib.
"They know your a New Mexican when you choose restaurants based on their salsa."