You should refer to the boxing organization rules. For example, this is from the WBO rules:
The Committee shall determine if the Vacancy should be filled by a contest between the two Best Classified Contenders Available, or a box off among the four Best Classified Contenders Available. In either event, the contestants as determined by the Championship Committee shall be granted a period not to exceed thirty (30) days to negotiate a contract for the vacant World Championship or an Interim World Championship, as the case may be with a duly qualified promoter
The WBO President may, upon the request from either contestant in a championship or any of the contestants in a box off for which a purse bid has been ordered, order a purse bid to be held within ten (10) days from the date of the request.
In the event that the contestants fail to negotiate the contract within the 30 day period, the President may also order a Purse Bid for that Vacant or Interim Championship, pursuant to the procedure established in these Regulations.
So the committee might have decided or maybe the #1 fighter could not reach an agreement.