Low-level question about OpenPGP - why CFB mode?

Why does OpenPGP use a custom CFB mode instead of CBC mode?  CFB with the
slide equal to the blocksize is basically different from CBC in that the
encryption is done before combining, rather than after.

I presume its simply always been that way, and it's not a problem so it was
never changed in an update.  But why was that chosen initially, if anybody
knows, and are there any propblems with CBC that this avoids?

--John