I suspect it may have a lot to do with attitudes and how they have changed over the years since the first book. The Colour of Magic came out in 1983. Attitudes to casual racism were a lot more relaxed then. Things have changed, partly due to people realising that racism is stupid - especially when you get to know the people you may have been racist or homophobic about, and realise that they are more like you than they are different. One of the things that Terry uses the Watch for, is to point out that all the members are, first and foremost, coppers. Vimes himself can be somewhat racist in the early books. Ankh-Morpork is becoming a much more cosmopolitan city and the citizens are learning to respect their fellow citizens. There are still problems - the basis of which form the plots of several of the later books, but they are getting there. The Discworld is opening up and people are relating to each other. It's not unlike our world now where experiencing other cultures and getting to know people from around the world via the Internet, is changing how we feel about people.
I remember that scene from Jingo, where Vimes takes Colon aside to explain to him why terms like "towelhead" are not diplomatic, and then Colon's discussion with Nobby afterwards about the importance of names. The thing is, Pratchett still makes Colon a likeable guy without condoning what he does. That's no small feat, considering Colon is essentially an idiot know-nothing-know-it-all coward with surprisingly bigoted views of other races and species.
If that doesn't sound suspicious right away, I don't know what is... 
