Because of the usual reason: They are too computer illiterate to run Bitcoin.
I don't know if we can jump to that conclusion. A lot of their donations will be for tax write offs. With using bitcoin i think there is a lot of upfront work they would have to do.
Do i think they would be smart to use bitcoin yes. Hopefully in the future we see them and many others adopt bitcoin.
The Red Cross may be doing something with Bitcoin. From CNN:
"A new bitcoin wallet is collecting donations for the Philippine Red Cross. ... Canadian startup Coinkite, a Bitcoin banking service, is also collecting Bitcoin donations for the Canadian Red Cross for Philippine typhoon victims."