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

[Ryan Dino]

No: HPFGUIDX 68002

I'm just curious why Dumbledore doesn't recruit the entire Hogwarts 
faculty to become members of the order.  I'm sure he's convinced most 
of the faculty about Voldemort's return and has their support but 
wouldn't the order benefit by having the other teachers as members?  
The only ones I know who are members are Snape, McGonagall, and 
Hagrid?  What about Trelawny, Madam Hooch, Professor Binns, Sprout, 
and especially Flitwick who seems to be quite powerful?  What about 
the other teachers who teach Arithmancy, Astronomy, Ancient Runes, 
and Muggle Studies who are seldom mentioned if ever?  Harry had so 
much trouble finding a faculty member who was in the order and had to 
go to MoM himself.  This wouldn't have happened if more faculty 
members were in the OOP.

Do you guys think there is a reason that they're not members of the 
order?

"Ryan Dino"