I remember people using that term in the 50s Falconbird.
I have a few theories, and they are only theories:
1. GPs are trying hard not to give antibiotics for sore throats and saying things like "come back in 7 days if it is not better". Most Sts are viral but SF starts with a bacterial (strep) throat.
2. In some areas of the country GP services are under such pressure that parents may be having difficulty getting an appointment. Throat then gets worse and becomes SF.
3. Poverty. Other bacterial diseases, notably TB were steadily declining in the years before antibiotics were discovered. There is a correlation between TB and overcrowded, impoverished living conditions. There may be an increase in families living in overcrowded conditions (changes to housing benefit) and a increase in families who don't have enough to eat (food banks...) that is behind this increase.
In NZ the incidence of rheumatic fever (that nasty complication of SF, which affects the heart) is higher in poorer communities particularly Maori or Pacific Islanders who tend to have worse health outcomes than white NZ-ers on many measures AND more poverty and overcrowding. Life expectancy of these groups 10 years less than white.
I just guessed that the same picture might be found in Australia and blow me down, I was right. (indigenous ethnic groups 20 yrs less life expectancy...)
www.rhdaustralia.org.au/about-arf-rhd/epidemiology