I will hold my judgment until more information is known. Depending on where the graduation is being held, it may be that the school administration feared a huge crowd coming just to hear the Governor speak-—including people who had no connection whatsoever with the graduation-—thereby causing family of students actually graduating not being able to get in. Media would also be prominent, and they may have wanted to avoid that, too. Relationships between mothers and daughters can be dicey at times also; perhaps Bristol told her mother that she preferred her not to speak such that “mom” would be the center of attention.
There could be a number of reasons.
Reads like partisan politics to me. Sad, really, but I'm sure the Guv will relish celebrating with Bristol and the family.