Yea it’s for sure popularity based. I wouldn’t honestly be surprised if he’s ahead of Leon and even Cynthia honestly. This isn’t exactly a statistical measure at all but when the M8 was announced, the JP twitter posted each individual champion and Steven had by far the most likes. It went Steven > Ash > Leon > Alain > Iris > Cynthia > Lance > Diantha with Steven being almost 2000 likes ahead of AshSimple answer : popularity, Steven is the 3rd most popular champion in Japan (very close to Leon for 2nd) and sometimes they just want viewers to watch their show you know.
Cynthia is technically more popular than Leon but they just couldn't put Cynthia 1st and Leon 2nd for huge narrative reasons and Galar bias so they just switched instead.
So yeah they just made their top 3 ranking by popularity , nothing too complicated.
Might not mean much but it is a pretty decent look at the excitement by fans regarding Steven’s presence in the tournament. Not that surprised if the ultimately gave him priority over Lance and Diantha when it comes to Ash opponents just for that