It's been mentioned many a time since vanilla. Here's a statement by the lead producer for WoW, but if you want to call him Chuck Norris - that's cool too.
http://www.tentonhammer.com/node/73567
IIRC, it has to do with how the background databases are set up. Adding fields and tables to existing tables is much more a feat when you're talking about thousands of tables talking to each other and containing information for so many characters.
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