Why are most BLDC motors designed to operate on low voltages?
Arguably, brushless DC motors are not strictly speaking DC motors. They are really synchronous AC motors in disguise driven by a variable frequency driver (VFD) with some tight feedback. 

Though they can be, and sometimes are, driven with large voltages, that adds some awkward side effects like, increased switching requirements, more electrical noise, and dangerous DC voltages.

Since you end up needing to down-volt to drive your control system anyway, it makes more sense to use motors down in that range also. As such there is higher demand for lower voltage motors than high voltage variants.