Business class initially was full fare coach, a way to reward people who were buying full fare tickets, perhaps because mulitcity routings were needed in the days of TATL RT fares all requiring advanced purchase and seven day stays. Revenue management was in its infancy and there was pushback as passengers started to realize the range of fares being flown on any given flight, with multiples of three being common rants then. IIRC SAS was one of the early ones, then TWA and BA with others following. IIRC business class appeared in the early 1980s.