First of all, let me say I'm guilty as well...
Don't you know that Ab or Db to organists is what C and Eb are to keyboardists?
My theory is this: the keys of Ab and Db has more black keys than any other keys, which makes it easier to accomplish the "heel-toe" method... I mean who likes to play shout music/congregational music in on the organ in A? Not to mention, doing runs on the organ (using the major scale for the most part, and I guess the blues scale as well) are much easier to do in those keys... I mean why do you think Tye Tribbett picked those keys for "I still got Joy?"
As for my keyboard analogy: Alot of Keyboardists love C and Eb because of the same reason: the pentatonic (blues scale without the b5) in Eb is all black keys(and in C, let's just face it, the pentatonic easy to play), major scale riffs are much easier to accomplish, and last of all it's easier to TRANSPOSE lol!!! Everyone knows that -4 is Ab and +3 is Eb(when you're playing in C)!!!
And also, alot of keyboardists(who don't play organ), don't know how EASY it is to play in Ab on the organ... and how DIFFICULT it can be to play in C on the organ.
But you know the weird part