ECM were good at promoting artists back then as well. I would imagine that artists on ECM reached a wider audience that they probably would through smaller labels.
I've just been thinking of some of the leaders/groups i've seen live when they were on ECM at the time of the concert:
Jan Garbarek, Dave Holland, Terje Rypdal, Keith Jarrett, Ralph Towner, Glen Moore, Eberhard Weber, John Abercrombie, Steve Swallow, Bill Connors, Arild Andersen, Colin Walcott, Kenny Wheeler, Gary Burton, Pat Metheny, John Taylor, Paul Motian, Gary Peacock, Don Cherry, Nana Vasconcelos, Mick Goodrick, Miroslav Vitous, Charlie Haden, Ed Blackwell, Egberto Gismonti, Chick Corea, Shankar, Lester Bowie, James Newton, Oregon, David Torn, Marc Johnson.
I've lost touch with what ECM do now but that was an impressive roster back then.