Because it's what people are used to. From experience I know that switching between US and UK layouts is rather painful: the key sizes are different, some keys

Because it's what people are used to. From experience I know that switching between US and UK layouts is rather painful: the key sizes are different, some keys are in different locations. One downside of having a "special" layout than everyone else is that everyone else's computer is harder to use (and also, harder for others to use your computer).