Paella actually has a lot of variations all over spain, and it was a very "what do we have on-hand" kind of food!

My relatives in Madrid have chorizo, shrimp, shellfish, rabbit, chicken, sausage as far as meats I've seen in the paellas they've made.

Chorizo, shrimp, shellfish, and chicken is by far the most common they make. It may also make sense that the paella that "caught on" in the states just hails from a different region of Spain. So "American Paella" may just be closer to a non-valencian paella.