The Manakov modulation instability is shown to determine the performance limit of long-distance Brillouin fiber sensors utilizing orthogonally-polarized pumps. Numerical simulations of the Manakov equation are supported by the experimental measurement of a 25-km Brillouin fiber sensor to verify the model. The study reveals that the use of orthogonally-polarized pulses decreases the pump depletion due to modulation instability leading to an enhancement of the sensing distance.