Well you have to ask why is Ash losing minor side tournaments? He won the DP tag tournament, both Pokeringers.
The only other side tournaments he lost were the Whirl Cup because it was obviously intended to give Misty some last bit of spotlight before she was removed from the show, and the Contests he was in because he obviously wasn't going to win those either.
I think that you've answered your own question here, really. It all has to do with the spotlight. Ash won the Pokeringer competitions because the writers thought it was fitting (and perhaps, for the second, a halfhearted attempt to say that Paul isn't the best trainer at everything ever, and perhaps also because they couldn't be arsed to think of another way to evolve his regional bird). Ash and Paul won the Hearthome tournament because it furthered that particular subplot; their rivalry was in the spotlight. And just like you said: Misty defeated Ash in the Whirl Cup (let's ignore the particulars of Ash battling with an uncharacteristic level of idiocy, even for him) because she deserved the spotlight, and Ash was never going to outright win any Contests because hey, those were May and Dawn's thing.
I'm not entirely sure if you've realized this (and forgive me if indeed you have), but this is the first saga in which
every major character--which is to say, our trio of heroes and each of their rivals, possibly excepting Burgundy for the moment--could potentially be seen as being pretty much equal. Not like previous sagas, where certain rivals (i.e. Paul, Gary) were clearly about a thousand leagues ahead of everyone, their respective rivals always in second place, and everyone else about a thousand leagues further behind. And guess what? The writers simply
cannot portray everyone as being more or less equal and give all of them appropriate battle time by having Ash win every single one of these tournaments.
That's why Ash isn't winning these things;
everyone is supposed to be getting significant in-battle spotlight this time around. Notice how it's mostly different characters getting the spotlight in this arc: in the first, it was pretty much Ash, Iris, and Cilan, with Luke as a one-off rival. This time, it's Ash('s Scraggy), Stephan, and Bianca, with this Masaomi guy as a one-off rival. And would you look at that: Ash is
still collectively getting more screentime than everyone else. But I guess that that simply doesn't matter because he's not winning the whole thing.
Now, I do think that these tournaments are leading up to something...but not because of Ash's performance in them. Looking at the paragraph above, I fully expect the spotlight to shift again for the next Don tournament, and to the characters who haven't gotten much through these arcs--specifically, to Burgundy, Trip, and possibly Georgia. Perhaps this will be where Burgundy manages to defeat Cilan in battle, and perhaps where Trip asserts his dominance over Ash one last time before the League. Can't say for sure, but it's what I'm expecting right now.
On a related note, I also think that the story is building up to something involving Ash facing Iris in a serious battle a second time...but not because Iris defeated him in the first Don tournament. In part, it's because of her position in the games; even though she's not even a gym leader apprentice at the moment, it's utter foolishness not to expect her and Ash to face off at
some point in the future--if not around when they reach Opelucid City or visit the Dragon Village, then around the tail end of the series. But mostly, it's because of the relationship between Scraggy and Axew. Playmates, battle partners...they've built up a friendly rivalry amongst themselves, and there's no way the writers are going to let that go to waste. I mean, I don't think I'm wrong to expect a Scrafty vs. Haxorus match for one of the series' final battles. Right?
