Because every other fucking champion in the game feels like a socially awkward nerd that never interacted with anyone until the VERY VERY END, along with their retardedly high intellect (that mysteriously gets outdone by some kid in some random hick town) makes them feel fake. They never really fucking do anything in the main story aside from say "hello" to the MC or helps the MC out with fucking GRUNTS of the enemy team. Furthermore, the regions those champions were in probably never viewed competitive pokemon battling as an actual sport, unlike in Galar.
>Steven
>Awkward nerd that doesn't have much friends, is smart and strong as fuck and enjoys collecting FUCKING ROCKS
>Lance
>Retarded chuuni that likes dragons "because they are cool"
>Iris
>Stupid girl that fails at everything she does, which is why she's probably the EASIEST champion in the series
>Diantha
>Know literally fucking nothing about this character, at all, until the very final fucking battle where "oh no she's the champion!!!!!!"
The only fucking champions in the older games that I can't fault are Gary - because he fucking acts like a champion - and Cynthia, simply because her character kept getting developed up to a point in the series. Doesn't mean it was a lot of (or good) development.
Now you compare this with Leon, a guy who is supposed to be fucking stupid and plays it risky, but has the talent in fighting to make up for his mistakes. This all paired with the fact he acts as the region's hero as well as the region willing to glorify amazing trainers and you got yourself a champion that actually fucking feels like a champion, because that champion has to ACT LIKE A FUCKING CHAMPION.
And last but not least, it's because no other fucking champion was actually hyped up in their respective games, they're always fucking hiding the fact they're champions. Leon didn't fucking hide the fact he was indeed THE MOTHERFUCKING CHAMPION, making hype surrounding his confrontation greater.
