It shouldn't be a matter of "porting" the number in, it should be a matter of just converting it to a Fi number. Did they give an explanation as to why? I would push for one, both services are run by Google, it should be nothing for them to utilize your GV number as a Fi number. In fact - if you try to use Fi on the same account as you currently have GV you have to decide if you want to keep your GV number as your Fi number, move your cell number to Fi and lose your GV number or get a new number entirely (losing both previous numbers).
I had GV on the account where I currently have Fi. I also have a Google Apps account so I moved my GV number there and ported my existing cell number to Fi.