Clive, I can tell you how....
It thrived for several reasons..
#1..... marketing.
#2.... studio accessability.
#3... catering to singles as well as couples ( something the UK never, by and large, did , on a priv. basis )
It was implemented to fill a void for many of the older generation with time ( and money ) on their hands.
Consider this,,, you are 60 plus yrs of age, widowed , divorced, whatever and very financially secure... now look at the time period.. the 40/50s , when it was unacceptable for ladies of a certain age, to be seen attending any social gathering sans partner.
It also provided an instant form of exercise coupled with the chance, for many , to fulfill a life time dream .
The downside to the premise ?.. it became pervasive with owners looking to make a quick buck.. not all, I might add, but like any bad publicity, it affected the whole " chain " system .
I worked for both on many occasions.. some pretty bad ones.. I also worked with some that had a business plan that was above reproach.
Looking on the positive side , it produced many world class dancers, who may never have had that opportunity to compete, were that system not in place .
As has been said before.. independant teachers do NOT provide any degree of capabilty, in fact, many are a liabilty.. I speak from extensive personal appearance gained over the yrs ( in ALL genres )