As it was an original plan for the stadium to be a 7 day open for business
There was an original plan to "sweat the asset" and have the Community Trust based in the stadium etc. but that proved unworkable with the latter design wear the focus was on saving costs and reducing risks. Therefore the new stadium isn't "open" 7 days a week, but neither is any other stadium AFAIK. However if you want your wedding there and the price is right then it will be opened.
The stadium is "open" in some respects as Box Office staff are there, groundsmen are there, and the club hold meetings in the lounges.