Blame Noah Webster.
Typesetters Quotations Versus Logical Quotations
There's another difference in how Americans and Britons treat quotation marks. In the U.S. we put periods and commas inside quotation marks, and in Britain they usually put periods and commas outside quotation marks. My admittedly U.S.-centric memory trick is to remember "Inside the U.S., inside quotation marks. Outside the U.S., outside quotation marks."
The reason for this difference begins with the introduction of movable type. Before typesetting, nobody paid too much attention to where they put periods and commas relative to quotation marks, but periods and commas became a problem with the advent of typesetting because they were so tiny. Printers found that the periods and commas were more stable when they were placed inside closing quotation marks, so that's the way they started doing it (7, 8).
Again, our British friend Fowler seems to have made the difference in his book The King's English. (9) Typesetting technology had advanced to the point where it wasn't necessary to shield periods and commas anymore, and he argued for what he considered a more logical system of letting the context of the sentence determine where the period and comma should go. The British seem to have taken his suggestion to heart and Americans seem to have ignored it.
Because of these origins, it is sometimes said the British use logical quotations and Americans use typesetters quotations.
Finally, you may be wondering why there are pronunciation differences between British and American speakers of English (not to mention Canadians, Australians, and others). The general idea is that regional and national pride and changing ideas about what sounded like "proper" speech, at least to some degree, played a role in changing the British sounding speech of the American colonists to what we hear today in America. It's far too complex to cover here, so I'll refer you to a PBS show called "Do You Speak American?" which talks about regional dialects too (10).
[Note, added 11/4/2014: It's also Noah Webster's fault that we pronounce huzzah as "huh-zah" instead of "huzz-ay." h/t @rosefox]
In summary, American English is different from British English because of the revolutionary leanings of a dictionary writer (Noah Webster), typesetting conventions, geographical separation, and the opinion of one influential style guide author (H.W. Fowler).
This article was written by Mignon Fogarty, author of Grammar Girl's Quick and Dirty Tips for Better Writing.
The Economist, a British publication, has an interesting page about Americanisms in their online style guide.
1. Lepore, J. "Noah's Mark," The New Yorker, November 6, 2006, p. 78-87.
2. Quinion, M. "Aluminium Versus Aluminum" World Wide Words, http://www.worldwidewords.org/articles/aluminium.htm (accessed July 2, 2009).
3. Quinion, M. "While Versus Whilst" World Wide Words, 18 May 2002. http://www.worldwidewords.org/qa/qa-whi2.htm (accessed July 2, 2009).
4. Woods, G. Webster's New World Punctuation, 2005, Wiley Publishing, Inc. p. 7.
5. Merriam-Webster, Inc. Merriam-Webster's Manual for Writers and Editors, 1998, Merriam-Webster, Inc. Springfield, Mass. p. 31.
6. Fowler, H. W. The King’s English. Oxford, Clarendon Press, 1908; Bartleby.com, 1999. http://www.bartleby.com/116/406.html#2. (accessed July 2, 2009).
7. pthompsen "Typesetters' Quotes vs. Logical Quotes" MacHeist Forum. http://cli.gs/pRSE2g (accessed July 2, 2009).
8. Wikipedia contributors, "Quotation mark," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Quotation_mark&oldid=299541669 (accessed June 30, 2009).
9. Fowler, H. W. The King’s English. Oxford, Clarendon Press, 1908; Bartleby.com, 1999. http://www.bartleby.com/116/406.html#1
10. Do You Speak American? PBS. MacNeil/Lehrer Productions, 2005. http://www.pbs.org/speak/transcripts/1.html (accessed July 2, 2009).