Why do humans hear better between 2kHz and 5kHz?
Acoustically, we hear better in the 2-5 kHz range because our auditory canal resonates in that range (specifically, around 3 kHz). Our ear looks something like this (U Miami):



The auditory canal is part of the outer ear, which amplifies sound so that it can be converted into electrochemical signals by the middle and inner ear. We hear better in the 2-5 kHz range because the resonant frequency of the auditory canal is in that range.

The mechanical question of "how does this work" is usually easier to answer than the "why did this evolve?" question. Since reproductive fitness is the driving force of evolution, hearing babies crying (between 1-5 kHz) is the best guess I can come up with.