There's an idea floating around that Missouri is waiting for the A&M deal to become official to take political heat off A&M from their state legislature. After A&M is officially in, Missouri follows suit.
This is speculation, but it's pretty good speculation imo. A&M will be under significant political pressure if it looks like the Big 12 might blow up. By waiting, Missouri provides them the pretense the Big 12 is still relatively healthy and then they join after the dust settles.
Once again, this is only speculation, but given the situation in Texas I think it makes a lot of sense. It will be interesting to see how this plays out.