Apple loves to have something for next years ’big thing’. Could just be their marketing strategy too. Getting people to buy more expensive 12.9. Reading some MR comments, it’s working.
Yep, while I doubt it is the main reason (my best guess is that it is just simple supply chain constraints), the fact that it could also get 11in owners to upsize to the 12.9in isn't a bad side effect from their perspective.
And before someone else mentions it, the other side of the argument would be that if you go back into Apple's history, there is always the exemple of the infamous "new iPad" (the first iPad with retina display) which was refreshed after only 9 months. But I think times were different. Back then there was only one iPad model, and this particular generation was a strange compromise trying to tweak the iPad 2 for driving a Retina display as a stopgap while waiting for a more proper chip to become available. This lead to display lag, slightly excessive heat, and stronger than usual battery drain, meaning there was an actual problem for them to fix.