Technically Fergus's answer which got no votes at all till my arrival was the most correct answer to the particular question asked; Lower notes have longer or heavier reeds, and thus more momentum to overcome for bending.
That is only half the answer however.
Bending is not about volume of air, over-blowing, or adjacent reeds as was mistakenly imagined for a century. Fortunately some great YouTube videos are demonstrating the correct concept these days, and how to easily apply it.
Bending is about resonant frequencies. More specifically one shapes their mouth cavity until it has a size which resonates with the fundamental frequency of a reed (or even one of the harmonic frequencies of that reed). Once you have latched onto the reed with a resonant mouth cavity, you can then reshape your mouth cavity such that a slightly different frequency resonates instead. - Harmonica bending is essentially the same thing as Tuvan throat singing, except that the vibrator is outside and adapts to the changes in resonance.
Now, back to the answer. The easiest bending is where the resonant force is strongest, which is up near the front teeth where the reflections are shorter and the helmholz resonator cavity is tighter. Lower notes have longer wavelengths, so where the tip of the tongue was sufficient to bend high notes, one would have to reform the back of the tongue instead for a harmonica in a lower key.
As with singing, there are several resonant centers in the human windpipes, and all of them can have some easy useful effect on harmonica tonality. There are two similar concepts taking place in a harmonica. One is bending, which is to latch on to a tone and shift it. The other is resonant amplification in which one simply strengthens a tone or any of the higher harmonic tones which a reed produces.