My crack theory is that it's Chief Director Yuyama Kunihiko who makes the decree but I can't say for sure. It might even be something forced down from Game Freak, although I'm not sure how much they care about the cartoon. My guess is that Yuyama has an old fashion idea that if Satoshi wins the League there is nothing left for him to do...but that's why they have the Champion League and can just introduce a National League (for Japan) and a World League. Satoshi would be able to 'advance' on paper but essentially be stuck losing to hundreds if not thousands of other trainers for the rest of eternity.
Someone definitely made this sort of "decree", probably for this old fashioned idea that if Ash wins.... but really, once again... writers/directors or "what else"... we have elite4, there is this champions league, we know that pokemon master isn't just winning a regional league but it's something else conveniently vague.... but no, writers still love their "comfort zones". After 20 years, we need something to restore some hope and expectations for the league itself.
My personal "story" with the leagues, across the years:
- 1° defeat, Kanto: many years ago... well, i was sad like all but kinda surprised for this loss just at Top 16 (and like many I expected an Ash vs Gary back then), and i sort of "liked" this unpredictably (but after 18 years... lol), a character that doesn't "automatically" win in the end for the "mere" fact to being in the "main cast" was "good" to me, and Ash was still a beginner afterall. So understable.
-2° defeat, Johto: Good to see (...finally!) Ash vs Gary at the league, sad by the loss against Harrison but still understable, a good fight and Top 8, so a... progession afterall, if we consider even the win of the orange league before, although it's a minor competition. It was... enought. Still liked this (apparent...) unpredictably.
-3° defeat, Hoenn: Start to think about, writers... do you really... not want... to let Ash to... win something? another Top 8, no regression but no progession... but at least, after that, the Battle Frontier win give me some hope for the future and for the next adventures in Sinnoh. So "ok", a bit "tired", but still want to... believe.
-4° defeat, Sinnoh: It was awesome Ash vs Paul and all the relative "storyline" in DP, but aside that, this was a sort of turning point for me, It is """""now""""" (lol) clear (even for someone "blind" like me lol) that writers love to "joke" us about this dream of Ash, namely winning a league. Writers introduced this champions league (and when Flint tells Ash about the time he won the Sinnoh League)... basically for... nothing ... Tobias was really indicative of this ""determination"". And even the pokégirls (May before, now Dawn) in the end... fail to achieve their respective dreams. A positive thing was the progression in Top 4, but... really tired by this pattern
-5° defeat, Unova: Still recovering after DP I didn't have any particular hopes in this season regarding the league, and Ash didn't give me the vibes for a victory afterall. Sad for the Top 8 but the ending gave me at least some hope, back to pallet town Ash was determined to win this time the next league in Alexa's land: Kalos.
-6° defeat, Kalos: Ash's determination, awesome gym battles, Ash vs Sawyer and Alain... but...I need to say...? very sad in the end. This time I really believed for a final victory, for many reason and i wrote them in this forum during the XY and XYZ period. One of these was Ash-Greninja, this strange bond-phenomenon, writers didn't give such "special things/powers" to Ash before. For this league and the relative defeat there is already an abundant "literature" on this matter, so I will not digress further. To be more positive, at least we have a final (after so many top 8 battles) , a Top 2 achievement... so a progression after... all.
- ....? Alola? Well, after Kalos I will wait and see for some time. I don't have a clear idea about this yet, the alola league, at least for now.
In the end I still have some hope, maybe someone will broke this "old scheme". In very-very broad terms, there is a (very slooooooow) progression afterall in these years. I just hope I dont' have to wait another 2-3... decades lol.