I don't know, but my guess is misplaced priorities by athletic directors followed closely by a lack of imagination.
What is a rivalry? I have no idea. I would say that UNC and Wake have no rivalry. Wake really has no rival. UNC has Duke and NC State to hate. UNC and Wake have decided that playing each other in OOC games when they are not scheduled for an ACC match up is good for the players, students, alumni, fans and general community. The last time they played it was a beautiful 6 PM, Friday night kick off in W-S. It was a great game with Wake having to hold off a desperate second half comeback by UNC. After the game, the bars and restaurants in uptown W-S were packed with smiling fans from both schools. Isn't that what Toledo and Akron should be trying to accomplish with an annual game.
What is in it for Toledo some ask?.?. Some snark there would be an easy win. Maybe, maybe not. The all time record between the schools is 12-9 in Toledo's favor. Two of the best games I have ever saw at the Rubber Bowl involved Akron kicking the longest FG in school history for a last second win and Toledo Tom's final season when the Zips won a well played game by both teams.
Maybe the question isn't, what's in it for the Universities of Toledo and Akron, but what is in it for the taxpayers who support these programs? What is in it for the players, students, fans, alumni and general communities for these two schools? Are they not intertwined? I would bet there is a lot more in it for these two schools playing each other than a game against a school like Troy or UNCC or ECU or Purdue or some other school people in Ohio have almost zero interest.