Large AAA games = important, huge games with a large budget and development team, but also huge sales. These are videogame bestsellers and the core focus of most large videogame publishers.
From SE's perspective, it makes sense. Next generation consoles should be coming in 2020, so it's really about time to start working on next-gen games as well. The XV team already had the next game in the pipeline, but they couldn't work on it fully as long as the DLC was there too.
The only weird thing here is that the management originally greenlighted the extra DLC set, only to realize that developing it will actually take time half a year later.