See I had very little idea of who or what the Thirteen were when Cybertron was current. Just that it was a thing somewhere and maybe important.Making all of them Primes all traces back to the naming of Vector Prime and his lack of carrying a/the Matrix, years before the 2007 movie's success kicked off the Binder's development.
So when Vector Prime showed up I assumed he was a past Matrix bearer who'd time travelled to help out the present day Autobots for reasons (something something Unicron something something Primus something something darkest hour).
I didn't make any connections to the Thirteen at first, much less assume Vector meant they were all Primes.
Had there even been just a throwaway line about Vector being one of Optimus' distant predecessors as Prime all of this could have been avoided.
Or who am I kidding? They'd have found another way to make it dumb.