I don’t think I heard the term “wintry mix” until fairly recently (within the last two decades at least). ‘Sleet’ was always one of those words I saw in print more often than I heard anyone actually use it, and I wondered what it was as a kid. Its lack of popular use (compared to ‘snow’, ‘hail’, and ‘rain’) and uncertainty of meaning may be why ‘wintry mix’ has come about here.