Overwatch has actual competition now, so the giveaways are promoting players to keep playing Overwatch as much as they are promoting a specific mode.
I was somewhat understanding for the first set of Stadium exclusive drops, but at this point everyone has tried Stadium and had plenty of time to decide if its for them or not. There’s no need to keep tying rewards to the mode. Similarly, if new content releases for non-Stadium modes it would be messed up to force Stadium die-hards to play modes they don’t enjoy.
Keep in mind it’s the launching season; they’re going to promote it. The same way last season every week there was an event challenge to get 5 loot boxes, and now it doesn’t exist. They’ll likely balance out the loot box attainment through other modes next season.