Photography was definitely 'in' during the mid-70's. Lots of magazines, lots of Arts Council funding. Creative Camera was interesting then - one of the few publications in my collection to have stayed with me when I moved to California.
I think the general interest magazines were hit by the microprocessor arrival. Personal computers really took off in the early 80s, and that probably attracted a lot of people who had a gadget fascination. Now the serious programming (leaving out Usenix' ;login, ACCU's Overload, and other non-newstand publications) and computer hobbyist magazines are giving way to gaming.
Fashion, I think.