Think about it. Maybe nintendo wanted to give Aonuma a little more time to have the perfect game and swapped the launch of mario and zelda.
Pokemon GO was released in July and it increased people's interests in Pokemon again. 5 months later: Pokemon Sun and moon which are the most preordered games on the holidays.
Now we have MARIO RUN confirmed for a december 15 release. Millions of people are signed up to be notified when it arrives. 4 months later: Switch with Mario title. It will drive up the sales of the system and the game all together, just as how the 2ds sales rose 500% when pokemon go came out.