Question

why do we prefer a potentiometer with a longer bridge wire?

Answer 1

It is done in order to increase the sensitivity of the potentiometer. Smaller the potential gradient, greater will the sensitivity.
One way of increasing it is by increasing the length of the bridge wire

Answer 2

Potentiometer works best when potential drop per unit length is less.

If you increase the length of wire, potential drop per unit length decreases and it increases the sensitivity of the potentiometer.