It seems to be because they don't know how to write a show centered around a league champion. It sets up a precedent that would be hard to ignore or just sweep away.
What would they do with ash if he won a league? I've seen the "champion's league" brought up. So, then what? If he beats all of a region's E4, wins, then beats that regions champion, then what?
Now, I know people will say the likelihood of these things happening is unlikely, but just consider where I'm going with this.
So, if he beats a regional champion then what? Does he go back and challenge older regions leagues? Or should he go straight to a champion's league tournament like in BW2?
It seems like Ash's league rivals are "last minute" because the writers have never intended for him to win a league, which would explain why they tend to be poorly developed, which then tells me that the amount of time they spend mapping out league "rivals" is questionable at best. To me, if they intended for Ash to win a league and to a lesser extent fight good rivals, a little more effort and maybe a little foreshadowing would go a long way.