Don't know, but they must have their reasons. Likely one is that they need a new stadium but cannot afford it. Sainsbury will finance the new stadium in return for the current site.
A similar thing happened here when Asda wanted the town centre site occupied by the swimming baths. In return for the site they built a massive Leisure Centre with baths, football stadium, gymnasiums, and a host of other facilities. These were far superior to what existed and what could have been provided without them financing it. So everybody won. They went from a small baths that was falling down with no money to repair (about to close as unsafe) to this:
https://www.google.co.uk/maps/place/...465ff269?hl=en
The new location could also be better for access, crowd control, and less impact on the local community.