IMHO, they didn't want to bring in the romantic element early in the series. So much money yet to be made through games, films, series and whatnot! Just look at the rate at which Ash matures! I feel this is the most important reason- all others are secondary. They can't keep postponing till the end of time (or can they?!). It will be interesting how they decide to do it. It is my hope that somehow he gets to be with Misty in the end!
Of course they didn't bring it in at the beginning. With how many kids were watching that damn show, they weren't about to screw up and add the romance then. Meanwhile how they'll do the romance in the series? They won't. Ash is maturing beyond normal because new writers. And one thing new writers will most certainly not do is bring back Misty.
Why not? Because not only does she have no relevance whatsoever in Kalos or Alola, but she's also been axed almost 15 years ago now. And in those 15 years (unlike the forever 10 year old Ash) grew, matured, changed. And since she's probably (according to how old Ash would be IF HE AGED) about 16-17 years old. Ash is still 10. The writers WILL NOT make something as horrifying as that happen in what's known as a kids show...
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Now so this post doesn't get deleted by mods, let's talk about the subject. Yeah, Misty was neglected compared to the other poke girls, especially May and Serena who received character development far beyond that of the rest of them. Misty was most likely neglected because, like Iris was in the Best Wishes saga, they didn't really know what to do with her.
Like in the BW saga when Iris was trying to be a dragon master, in my honest opinion, the only real development she ever got was when she was trying to train Dragonite with the oh so overrated Cynthia. And what's funny was this was during the time Dawn was there, so she's the only one of the three poke girls without a good reason to return, or have any sort of final character development like May did.
Misty in her goal of becoming a water Pokemon master, got little development. In fact, I wanna say that almost 90% of her meaningful development happened in Pokemon Chronicles, and in Hoenn. And guess what? Most of that was still in Chronicles. I think they put her in Chronicles because they knew that she needed some character development before she was axed permenantly and had no appearances in any aspect of the anime whether it be a spin off or the main series.
Overall, yes. I believe Misty was extremely underdeveloped in the days when she was a part of the main cast, and even less than that of Iris (which is saying something), and it's a shame that such a well done, and universally liked character had virtually no character development when she had the opportunity.