Meh, there was one semi-decent reason in ESB:
QUOTE
LUKE: Oh, no. We'll never get it out now.
Yoda stamps his foot in irritation.
YODA: So certain are you. Always with you it cannot be done. Hear you
nothing that I say?
Luke looks uncertainly out at the ship.
LUKE: Master, moving stones around is one thing. This is totally
different.
YODA: No! No different! Only different in your mind. You must unlearn
what you have learned.
. . . . . .
LUKE: (panting heavily) I can't. It's too big.
YODA: Size matters not. Look at me. Judge me by my size, do you? Hm?
Mmmm.
. . . . (after Yoda lifts the X-wing)
LUKE: I don't...I don't believe it.
YODA: That is why you fail.
The implication here is that the Force works off of belief, and does not obey the laws of physics. Lifting an X-wing is not a matter of raw "power", but concentration and understanding.
If this is the way things work, it might well be easier to train small children to use the Force before they learn any preconceptions about the way things work. The fewer concepts like "big things are harder to lift than light things" you've learned, the less you have to unlearn in order to think in a Force-centric manner.
'Course, this concept is largely torpedoed by the whole "midichlorians" thing, so those interested will have to come up with something else now.