Continuity mistake: During the calibration with Mako and Raleigh, Gipsy's angles change between shots. (This takes place before and after Newton explains what happened when he drifted with the Kaiju).
Other mistake: When Crimson Typhoon's head is pulled off, their location should not be visible on the radar behind Stacker.
Continuity mistake: When the three Jaeger team are taking on Otachi and Leatherback, Striker is behind Cherno and Typhoon during the fight. However, Striker is in front of them on the radar as Stacker tells Striker to not engage. Cherno and Typhoon were in the same positions on the radar and in the real fight.
Other mistake: The war clock during the triple event does not reset as it should when a Kaiju breaches.
Continuity mistake: When Hermann and Dr. Newton are about to drift with the baby Kaiju it is night time, when Striker Eureka and Gipsy Danger are being transported to the breach it is daytime. Even though they are both in Hong Kong they are in different times of day.
Continuity mistake: When Gipsy Danger is taking on Otachi, the acid-spraying Kaiju, they use the chain sword when they're in the air. However, the computer says it is deployed before Mako actually activates it.
Continuity mistake: When Gipsy defeats Leatherback, it's covered in the Kaiju's acid. When we see Gipsy again trying to defeat Otachi, it's clean.
Audio problem: When Dr. Newton and Hermann are with the baby Kaiju, Hermann says "Should be three," but his lips aren't moving at all.
Chosen answer: Most likely they only use it when they know they have a guaranteed shot and won't waste it. It can't be reloaded that easily. Like having one bullet to kill a zombie vs a sword so go for the sword save the bullet it's more valuable. That's typical military tactics. Possible firing whatever weapons could cost a ridiculous amount of money just like missiles do. It's not exactly like Yeager weapons were mass produced they are all different. Last reason is reducing extra casualties by making sure they don't miss when they fire or making firing weapons the last resort. And just for the movie's sake it would be boring as crap if they just blew the heads off in seconds.