(in best Andy Rooney voice)
That it's only a certain type of surname that was usually handed down as a fn? Beverly, Ashley, Sidney, Cooper, Tucker? You never see people with the first name oh... Feldman, Rivera, Lefkowitz, Shucmacher, Lefebvre, Spinelli?
Basically it's a relatively small collection of "white Anglo-Saxon Protestant" type surnames that turned into first names.
This topic would make somebody an interesting research paper, no? :)