My guess: to slow down the schedules would derail the rest of the franchise. They likely already have the anime, the Trading Card Game, and other various aspects already planned out for the rest of the generation.
And they can't simply plan those to last longer? Pokemon has 1000 Pokemon now, there's no shortage of source material for the other arms of the brand. I don't think an extra year or two would derail the franchise at all, TPC is just being cheap and lazy here and I don't have much sympathy for this problem.
EDIT: Also Nintendo's not as concerned with the media and merchandising arms (I'm sure they want them to be successful, but it's not priority #1 for them like it is for TPC). They're a gaming company first and foremost. I don't think they want the games' quality to suffer for the sake of media and merchandise. They're even on record saying they'd prefer to delay their games than rush them out a buggy mess like this. So why are they just waiting until now (if even that) to address the issue?
Of course, that year when there was no main series Pokémon release (and we instead got one of the Pokémon Mystery Dungeon games), the Pokémon fans rioted, which gave these companies the message that the fans won't tolerate more than a year gap between new content for the main series.
Did they? I don't remember anyone complaining about no main series game in 2015. And it wasn't the first time that Pokemon took a break year (it was only the first in a while for international fans, the last being 2008). I wasn't unhappy with them taking a year off in 2015. I was unhappy with them moving on to 7th gen in 2016. It felt too soon, they rushed it for the anniversary and the game didn't really do anything to celebrate it, and we lost out on a sorely needed Z/XY2/whatever in the process. After 3rd and 4th gen both lasted 4 years, I expected 6th gen to (5th gen made sense to be cut short because it released during the DS' twilight years, but 6th gen had no such issue), but nope, they had to cut 6th gen to 3 years for questionable reasons.
Anyway, I don't think that was a justified complaint. A break year every now and then won't kill them. They don't really need to as much anymore because we have DLC and new styles of games to pad out the generations, but I don't see an occasional break year being some huge crisis.