Because the writer don't care for that. It's that simple.
Now for whatever reason, Ash not taking it that seriously more or less makes his whole Alola stay feel so barren that I could say it's just a vacation for Ash. And that's mostly the fault of SM trying to ride the SoL rails to a point the overall "plot" of Ash's progression feels non-existent. And since Ash is the only one in SM with a real goal that involves battles, it's breaking SM apart in terms of overall enjoyment.
Now people say: "But the series isn't all about battles!!!!" and while there may be some truth behind it, battles are still taking a big chunk in importance in the series. What does plot needs to decently unfold? Conflicts between characters/Pokemon etc. Now what are the writer using to create and finish conflicts in both the anime and games? In a series which people talk in form of their Pokemon clashing against each other, battles. It's taking a big part of the cake here in Pokemon. It may look differently in a SoL anime, but Pokemon isn't one, it's a Shonen anime with a focus on battling. But that's a talk for another day...
Now returning to the actual subject here: Ash not taking his "quest" in Alola seriously and those trials barely getting mentioned more or less give me the feeling that they don't know what the hell they want to do with them and Ash in generell. And since the other class mates don't have any decent goals, there's a lack of substance in SM. If Ash doesn't show that much interest in them, then I won't be either.