According to the article, they did try cleaning them...
Supposedly, some not to bright individual actually tried to send the Apollo 11 suits to a commercial dry cleaner. I don't remember if they actually did it or if it was stopped before getting there.
Sure, there are still traces of moon dust in the suits that got deep into the fibers. Unfortunately, trying to remove that dust might further damage the space suits. At some point, historical preservation has to be considered over gaining a few more milligrams of moon dust.