HAHA!!! Yeah, well, there are certainly a few sopranos whose ankles you might NOT want to see, but...

Yeah, its "tradition" that only mezzo-sopranos can wear pants to auditions, because they have the correct voice-type to play "pants roles", which are basically roles of a man sung by a woman. (Usually they are from Baroque operas and were originally written for castrati, but not always.) Since sopranos usually don't have reason to wear pants onstage, they are expected to wear dresses to auditions.