Because they chose her. Really other than Alec Guinness and Harrison Ford the original SW cast was a bunch of nobodies and also rans. As for the “exclamation”, there are people who like getting less non-white faces on the screen. Her level of involvement makes as much sense as Mark Hammill who had 1 (one) voice roll pre-SW.
“Hamill’s early career included a recurring role on the soap opera General Hospital,[9] and a starring role on the short-lived sitcom The Texas Wheelers.[10] He portrayed the oldest son, David, in the pilot episode of Eight Is Enough, though the role was later performed by Grant Goodeve.[11] He also had guest appearances on The Bill Cosby Show,[12] The Partridge Family,[citation needed] Room 222 and One Day at a Time.[citation needed] He appeared in multiple television films such as The City,[citation needed] and Sarah T. - Portrait of a Teenage Alcoholic.[citation needed]”
https://en.wikipedia.org/wiki/Mark_Hamill
Peter Cushing was no also ran. :) But even Harrison Ford was pretty much an unknown when SW came out. He had bit parts in “The Conversation” and “American Graffitti” but the rest is mostly TV movies.