...Or Lucas said "screw it" and wrote her death. Trying to justify a major plot hole by trying to find some way of making it work is silly. Lucas wrote that originally because he had no idea what he was going to do in the prequels, and at the time, it was decent dialog between her and look as they talked about family and worked toward the reveal. Now it stands as a glaringly obvious oversight.
http://starwars.wikia.com/wiki/Queen_of_Alderaan
She also apparently lived until the destruction of Alderan, not that that was ever mentioned in the movies.