why isn't Unicode the default encoding?

[John Salerno]

Forgive my newbieness, but I don't quite understand why Unicode is still 
something that needs special treatment in Python (and perhaps 
elsewhere). I'm reading Dive Into Python right now, and it constantly 
refers to a 'regular string' versus a 'Unicode string' and how you need 
to convert back and forth. But why isn't Unicode considered a regular 
string by now? Is it for historical reasons that we still use ASCII and 
Latin-1? Why can't Unicode replace them so we no longer need the 'u' 
prefix or the encoding tricks?