Mutton really lost it's standing in WWII, when there wasn't the time to worry about what condition the animals were in before slaughter. For good mutton, you have to condition the animals up before slaughter.
    Lamb doesn't have the same quality issues, as the meat hasn't had time to become tough etc.

You can get mutton, but it's not that common. It's very good in curries, as the tougher meat means you can have a longer, slower cook than you can with lamb, and the flavours are much better all round...