Maybe it's the maple that produces a stronger vibration, hence the strong sustain and propensity to feedback. I guess maple is a harder wood than mahogany or rosewood, correct?? Thanks for posting the above info!
While acknowledging Pascal's input about density, I think maybe it's more about maple being recognized for having the "truest" frequency range reflection.
Backs are reflectors more than anything else, but
how they reflect is what gives 'em their characteristic "tone".
Mahogany and rosewood are known for emphasizing certain frequencies ('hog for midranghe and rosewood for bass) and related overtone chracteristics
Maybe maple's optimum reflection range is also well-matched to the optimum range of a dreadnought bodied guitar, and finally, that the arched back is known to enhance sustain and overtones.
Member Marcellis once said he believes maple records "the truest", and I think he meant the recorded sound is the truest to the sound of the actual instrument.
If that's due to minimal overtones and flat frequency response from the back, then building up a clean resonant frequency would be very easy, resulting in very good sustain due to absence of phase cancellation issues when various frequencies interfere with each other.
It's actually the drawback to an arched back, they can muddy up under 'hog at least, because the enhanced sustain tendencies of the arched back create more opportunity for interference frequencies.
Try to visualize the sound waves as a moire pattern, that might help.
I'm taking a wild guess maple would look something like this and thus be very stable and capable of long clean sustain:
'Hog probably looks something like this in an arched back body:
The whiter areas represent frequency interference/phase cancellation; the curved areas represent 'hog's ranges of frequency emphasis and overtones on top of the strings' output.