Main reason is to safeguard against piracy.
Super Mario Run will be one of the bigger releases this month since it marks Nintendo’s first real mobile Mario title. However, it’s worth noting that for all its single-player features, you’ll need a stable internet connection play it.
This was confirmed by Nintendo of America president Reggie Fils-Amie when speaking to Mashable. “We view our software as being a very important asset for us. And also for consumers who are purchasing the game, we want to make sure that we’re able to offer it to them in a way that the software is secure, and that they’re able to play it in a stable environment. We wanted to be able to leverage that network connection…in a way that keeps the software secure.”
Fils-Aime then further confirmed that this mechanism was in place to prevent piracy. Even launching the game on iOS first is a part of that and he also discussed why an internet connection was required. “We had thought at one point that it would be nice to have the World Tour [story] mode available standalone, to be able to play without that connection. But then the challenge is when that’s operating in a standalone mode, it actually complicates the connection back to the Toad Rally and Kingdom modes.”
Super Mario Run is out on December 15th for iOS and 2017 for Android.