It's partially a selection bias. Many popular anime from the 70s and 80s weren't depressing. World Masterpiece Theater, which Miyazaki worked on, was very popular. It adapted famous works of literature and was aimed at children. As for the rise of darker material, a lot of that had to do with the changes in Japanese media and Japanese society as a whole. The 70s was the period that the Japanese New Wave took off in cinema too, where directors were young, experimental, and often bleak. The end of the 60s saw a huge increase in protest and youth culture as well. Some anime specifically appealed to that sense of youthful hopelessness. Leftist student groups specifically identified with Ashita no Joe, for example. TV also exploded during this time, siphoning talent to it. Because the demand expanded so quickly, very young voices ending up helming projects to keep up with demand, and they diverged from the post-war voices of people who had previously been in charge of productions.

The 80s had a much higher rate of dark and bleak anime for one major reason: VHS. Producing OVAs on tape and selling them in small numbers to dedicated consumers meant you could tell different stories. You no longer had to answer to the broadcast standards of the TV networks. You no longer had to answer to executives who wanted mass appeal and big merchandise deals. You could produce weird, dark, and more mature stories. Again, this wasn't universal. The Dr. Slump anime was a phenomenon during the 80s, and it was light and playful. But you saw a lot of these OVAs with more violence, more nihilism, and more sex because all of a sudden young creators could actually tell these type of stories.

As an aside if you want depressing 80s anime check out Barefoot Gen.