Marshall left because they saw that the grass was greener. At that time, C-USA was ECU, Houston, SMU, Tulsa, UCF, Memphis, Tulane, Southern Miss, UAB, and Rice.
UCF, Houston, Memphis, Tulane, ECU, and Southern Miss have left for the Big East. Those schools have been replaced by what is essentially the old Sun Belt.
The MAC is better than what they are left with. They'll go where the grass is greener, just like they did before.