It all boils down to money and status, really.
ND doesn't have to share their TV money with anyone and not every team going indie is getting a $50m deal with one network.
They have enough pull and regional/historic rivalries to get the B1G and ACC to functionally do scheduling agreements with them so they get a good number of P4 opponents per year. Any team who leaves a conference to go indie has to find conferences willing to do the same, and it is likely that upending conference stability by going indie is going to cause the conference you left to refuse scheduling agreements (barring historical rivalries, like Texas would salvage OU, aggy and Arky, but everyone else would probably tell us to pound sand). Same with finding a conference home for your non-FB sports.
ND doesn't want to share their CFP money either - up to $20m that is all theirs
You have to do it before the CFP format changes and hope you get a built-in ND deal (10-2 gets you in, almost automatically)