Apparently the last Mario entry ins't only full of mushroms and question blocks but also... Splatoon Gear !
We currently don't know why this content is on the cartridge but I have a theory:
What if Nintendo, when they end releasing the content they have for Splatoon on the Splatoon disk, announces DLC you have to pay for, except if you own a Super Mario Maker copy ? knowing that th game will sell A LOT the vast majority of people by that time will already own BOTH games, so it would be really clever, as well as pushing sales for both games !
source:http://www.gamnesia.com/news/a-ton-of-splatoons-beta-elements-are-hidden-in-super-mario-makers-files