It's a result of cultural sensitivity. The Scots prefer not to refer to themselves, or be referred to as, 'Scotch'. Before the second half of the twentieth century, Anglophones tended to use whichever terminology they liked to adjectify people (Scotch/Jock for Scots, Indian for Native American, negro for black etc. etc). Nowadays people tend to be slightly more sensitive of the implications of their use of language and adjust accordingly. In that sense, it fits into the wider pattern of what might be called (though I dislike the term) politically correct language.