What are the possible causes of a faculty position not being filled?
You will probably never know the full reason, because departments are not quick to air their internal issues with job candidates. But here are a few possible reasons:


The search was halted by higher administration. One possible reason to do this so late in the process is because of funding changes (e.g. the funding for the position disappeared or was reallocated elsewhere). 
The search was halted by the HR department due to some violation of hiring practices. 
The search committee was unable to come to an agreement about who to bring to campus for an interview, or the dean rejected all the candidates before they were interviewed. 
Although this is less likely, someone who had planned to retire might have decided not to retire, or someone from another department may have been moved into the department (e.g. for legal reasons to settle a complaint).