So there really are only two possible reasons he would not come out and say definitively that he is staying put:
1. He really does want to move on to a P5 school
2. He wants to stay on at UCF but wants a much bigger salary
Actually there are two other reasonable reasons.
1. Even if he says he wants to stay at at UCF, no one is going to believe him, just like no one believes any successful G5 coach when he says he wants to stay at his G5 school. It won't make the talk go away, so maybe he decided that not addressing the issue at all (at least publicly) is the best way to handle it.
2. If (and that is a BIG if) Frost has decided he wants to stay at UCF, why would he want to publicly declare he has no interest in any other job this year (including Nebraska) which would embarrass his old school, his former team mates, and those fans who really want him to be the Huskers coach. He can just quietly turn it down if offered the job, and everyone can claim publicly it was never offered to him.