You'll have to ask Motorola that.
A guess: the thickness required by the new display reduced the amount of size for battery. Or, Motorola decided that 3760 is more than big enough to give 48 hour battery life for a moderate user. Or, anything bigger meant that the phone was either too heavy or too thick to also include the qi and powermat charging. Or, the custom motomaker cases prevent anything larger.
I'd say that 99% of people need only enough battery for heavy use to get from wake up to bedtime. And probably this phone does that.