Because it takes significantly more effort to get effect out of, is only effective at disabling vehicles as opposed to outright destroying them like the Sniper AND doesn’t have the fast TTK or Multi-Kill potential of the Sniper.
You’ll notice all the power weapons generally have extremely fast (usually one hit) TTKs (the Shock Rifle can actually lose vs. the Sniper if they both shoot at the same time) AND high multi-kill potential. The Shock Rifle is very much a slower TTK than the Sniper AND is a one-and-done weapon while the Sniper has the potential to OHK up to four people in a span of a couple of seconds.