My best guess is that since Mario is the mascot, face/Golden Boy of Nintendo, and the most popular video game character in the world, Nintendo and Tencent feel the best strategy for launch is to go full force, all in with him to get the system going strong. I'm not sure how known or popular Zelda is over there, but obviously not as strong as Mario. I honestly don't know about Smash Bros. and Pokemon though.
Pancho A. Ovies
Nintendo Switch in Japan (Famitsu): 2018 vs. 2019
PlayStation 4/Xbox One/Nintendo Switch: 2018 vs. 2019