Actually, 35 mm E6 sales started to drop in the '80s, when development and printing of pictures became less expensive and families started to use print film for their vacations. Until then there was a slide projector in every family and endless invitations to see the slides after a vacation.
Slide film remained a professional choice until the early 2000s, because it was easier to scan for printing in books and magazines with drum scanners. Then professionals moved to digital and nowadays it is by far a niche market than 35 mm. In some turistic locations finding even basic 100 ISO slide film is a nightmare.