Because he can kill pretty quickly. take the helix perk that increases accuracy during hailstorm and when his gun heats up and you can deal some serious crits. Stun an enemy with the dash and then pop hailstorm to keep 'em slowed as they recover from stun and hit 'em in the head. If it doesn’t kill 'em, it’ll leave 'em pretty close to it and scrambling back to base or their healer. I use montana more as a killer than being my team’s living shield. If you wanna be the shield, you can still interrupt with his dash and ult to save teammates.
(I also like to take the perk where using hailstorm reduces heat when used, so I can use it whenever my guns starts to overheat to keep up the damage and pressure. If I’m shooting to those levels of heat, there’s probably a reason, right?)