They weren't rare at the time they were designated to be the national bird. Their population was decimated by Monsanto's DDT, which rendered their eggs infertile/poisoned and their numbers still haven't recovered from that debacle. That's why they remain a protected species.
They were nearly poisoned to the point of extinction when some of us were younger. No one understood why at the time. Many birds like eagles, hawks, owls and condors were poisoned by the pesticide DDT. It was used to control Mosquitos and other crop destroying insects. The birds didn't immediately drop dead but instead the chemical thinned the egg shells making it almost impossible for them to reproduce. By the time anyone made the connection we almost destroyed the symbol of our country along with many other prey species. DDT is banned in this country. Enjoy watching those birds.