Why aren't the entire Hogwarts faculty in the Order of Phoenix?

[Steven]

No: HPFGUIDX 68077

I think you are missing one important fact that would be on 
Dumbledore's mind, which is making the school totally safe for the 
student body (in particular Harry). For all we know, the entire 
faculty is privy to the OOP, but are charged with protecting the 
school at all costs. It wouldn't do for all the faculty to be 
flitting about when there is an attack on the school, would it? 
Additionally, we only know those whom Harry has seen. For all we 
know, they are all members in good standing and Harry just hasn't 
seen them. Since, Harry is NOT a member, he does not know ALL the 
members.