>>279918127Back in the day, they had 26 episode seasons and would need to fill time with filler. There's also some things to consider when it comes to adapting a work like 1) some things in animated form don't work as well as a still medium like manga 2) some arcs might have had a negative fan reception in the original manga and so they respond to fan feedback by not animating it 3) sometimes as a show is airing, anime original characters or arcs get more popular so they start to lean into those, etc.
All of this said, there's obviously examples when it's just producers or creatives not liking something and just ignoring it. See Urusei Yatsura and Sailor Moon for big examples. But other times, the anime original decisions prove to work well. It's a crap shoot.
But the modern Ranma remake doesn't have this excuse. It has both the original manga and the first run anime to reference, as well as the fan reaction to both. And it skips some fan favorite parts from both. And if modern censors won't allow Ranma to be fully adapted, then they shouldn't have done a reboot to begin with.