I think one of the reasons is because of the importance people place on Ash's competency/success as a trainer, which strongly correlates with how much someone likes a particular part of the anime.
So let us look at Sinnoh and Kalos, the regions before Unova and Alola.
Ash peaked in Sinnoh. His battling skills immensely improved and he had a solid well rounded excellent team. On top of that team, he also used his powerful reserves in the Pokemon League. Combined with this, he had an amazing Rival who helped push Ash and take him to new heights and the anime was forced to introduce a Trainer with Multiple Legendaries as an excuse to defeat Ash as he was simply that powerful in Sinnoh.
Instead of peaking upwards, Ash regressed completely in Unova where he started behaving like a Rookie during battles. His Pikachu lost against a newbie trainer's Snivy. Ash even had an embarrassing defeat in the Pokemon League where he lost against a trainer with a disadvantage.
This downward trend in Ash's growth as a trainer was one of the biggest issues.
As for Alola, it again goes back to Kalos. The Ash in Kalos started battling again like his Sinnoh self. He was even given an elite four tier pokemon taking him one step further. His final battle was against a trainer who actually defeated an Elite four Member and Ash actually put up an even fight which was very impressive. Ash even managed to force a Regional Champion to take him seriously during battle.
After all this, Ash's competency as a trainer went more downwards than upwards here.
Yes, Ash won his first regional league and he was still a veteran trainer and his performance didn't fall as much as it did in Unova. That said, the main issue here was simply the lack of competition. The league Ash ended up winning was the same league Team rocket finished Top 8/16. There was hardly anyone as good or better than Ash in the league, to give him a noteworthy win. Gladion wasn't really that impressive or better than Ash, to begin with. The same could be said about Kukui.