Two reasons. One Stephen King is old (no longer on drugs) so his writing style changed quite a bit. Second because he likes these characters enough to want to reuse them.

Go back read any of his early work. I am been rereading a ton of King books some I haven’t read since I was a kid. You will notice that he brings up characters and shit from all his other novels. We all know about the Dark Tower and how he sprinkles it all over his books.

But he does this with all his stories. He brought up The Shining in Billy Summers. I am reading The TommyKnockers now and he brought up Cugo, IT. Hell he even brought himself into the story by having a character mention that horror writer up state.

Basically King as always if he can bring back characters our mentions them. Also why not dude a freaking writings machine and can write some of the most fucked up shit imaginable.

Funny thing King himself originally meant Holly to be a throw away character. She wasn’t meant to be but a side note. But as I writer I understand sometimes we don’t know ending we go with the flow.

So as he was writing she took control and in the end he feel in love with that character. I have to and I look forward to seeing were this goes. And I don’t believe any of the stories of this series be the same without her.

I also disagree that his stories are about the "badey or monster " no, they are about the people and without those characters the monster would be nothing. Like you said the Dark Tower would be nothing without Roland. Well Mr Mercedes, and the rest of the series would be the same without Holly.

Just my opinion as a Stephen King fan going back 35 years.