It has sold so much because it is so appealing. It is an entry into the most popular video game franchise ever (barring Pokemon), it's the first time in a decade and a half that this franchise has manifested in a 2D form on consoles, and it plays up the system's local multiplayer capability. Basically it's aimed at an under-served segment of the market and has a guarantee of quality, making it very appealing to people who otherwise cautiously spend their gaming dollars.
You do not have the right to never be offended.