Because the term "prejudices" embodies the idea of strongly, and often stubbornly held preconceptions, in a sense they are already "fixed". And so, in this case "fixed" is most applicable to "attitudes" and is, perhaps, redundant for "prejudices". But it does serve to reinforce the point.
However in: "Her cooking always involves the most exotic tastes and aromas", "exotic" would be read as qualifying both nouns, as konungursvia says.
not a teacher