I don't feel like Pokemon had completely forsaken continuity. Prior to JN, continuity references were rare, but AFAIK, the past continuity wasn't contradicted either.
This is a structural problem with the series. It's a marketing vehicle. Yes, it expands on the world of the games, but it primarily exists to sell the games and other merch. As
@Morax said, its formula is structured to allow the anime to keep its core characters while serving as that marketing vehicle.
It's also targeted to a specific age range, and the show doesn't change to keep up with the core audience. I feel like its designed to have audience turnover ever few years.
I think that these factors mean continuity references take a back seat to keep things focused on the current generation. We don't really hear about past companions unless the story would allow them to show up or there is a product they can promote. And a companion return has to fit into the storyline. How do you justify Iris being in Kalos or Serena visiting Alola while Ash is there?
Given the age of the series, I wouldn't be surprised if continuity wasn't referenced because it was forgotten. The show has been airing weekly for almost 25 years.
JN does change the format up a little. It's not locked into a single region, and this is the 25th anniversary of the games and the anime. I think that gives the staff more freedom to tap into the shows past. Overall, I think they've done a good job in bringing back past characters, but I can also understand why people don't like it.