It broke new ground in the area of visuals–especially in a sports game–and it's combat engine, while flawed was addictive. Add into that another significant achievement in storytelling with its Champions Mode, and you have the formula for a cult classic.
The Xbox 360 version received an 86 on Metacritic and an 84 on PS4. Both are much higher than average, and the game is still referred to in positively by its fans. After as one of the backward compatible titles on Xbox One earlier this year, there was a buzz almost akin to a brand new release. So the question remains, why haven't we seen a sequel?
Moreover, why haven't we seen any company (indie or AAA) release another boxing simulation? In this instance, VR games like Creed: Rise to Glory don't count because those titles rarely resonate with traditional gamers.