Circumstances change, the leverage of individual parties in the contract change, and there is no contract that can't be renegotiated.

That is the nature of business.

Why were (are) some of the MWC teams upset with BYU?  They felt that BYU broke the contract of the MWC when it went Independent.  But circumstances had changed and BYU felt it was in its best interest to leave the MWC.

And as much as it would be nice to have a ND-BYU game in Provo, it may actually be in BYU's best interest today to have it in Vegas.  Other than those in the athletic department, who knows what all the details are? 

But if you were the ND AD why wouldn't you look at and push for an option that would be financially better, more exciting for the players, and potentially land an even better tv slot?   Seems like exactly what you should do if you were the ND AD.