The 1979 anime version of Doraemon debuted during a time of transition for anime. Although Tatsunoko production were pretty much kings of the field, the Time Bokan series and in particular Yatterman known for humorous female nudity, and Go Nagai still influenced things heavily including an adaptation of the nudity-heavy Cutey Honey, 79 was also the debut Yoshiyuki Tomino changed the game with Mobile Suit Gundam. That anime was known for having a mature, almost adult-oriented story that involved the typically "childish" concept of giant robots, wrapping things up in a war scenario. Aside from that, the trappings were just like anime from the time, and a kid who sat home to watch Gundam would still be at home with Mazinger Z, albeit with a bit of a new direction and more eye-opening.
This, in turn, would usher in a sort of golden age of manga and anime. Stories were beginning to be told more for adults, and along with it, fanservice started to be used more for sexual gratification of the audience. This was the era where Zeta Gundam began including sexual showers of female characters, where City Hunter had gags of a similar kind, and female action stars like the Dirty Pair took over. And yet, all this was still intended for children, including violent works like Fist of the North Star!
But while that did mean a lot more fanservice for the older guys, popular stuff and kids stuff had to be a lot more careful about what they included. Superbook denied the young girl a chance to swim nude in the lake of Eden, a chance her male counterpart got. Bulma, who in the Dragon Ball manga got to show her breasts numerous times, had to censor them in the anime version, while 12-year-old boy Goku showed all of himself no problem. And while Panzer World Galient and Magical Idol Pastel Yumi did include some nudity, they still had to be censored at least a little. We came a long way from 1979, where even Gundam allowed female breasts, including those of children!
These trends, along with the anime, continued into the 90's. Even when the Doraemon manga ended in 96, the anime went up to 2005 with the same techniques. The 90's were the time where Ranma 1/2 could show female breasts, but Inuyasha by the same author could not. Shows began cutting down on nudity, but only to the point where it was reserved for adult works. And then, in 1995, Hideaki Anno got depressed and tried to destroy anime forever with Neon Genesis Evangelion, traumatizing children everywhere and making networks crack down on what was considered "acceptable" content in terms of all regards. It seems fitting that the fall of Doraemon coincided with the rise of Pokemon, an anime which played things much safer in terms of content.
The basic gist was that the concept of anime as "kids stuff" was starting to fall apart at the seams. More and more, kid's entertainment had started getting more and more restrictions placed on it. It became apparent that this older Doraemon series was obsolete...