Continuity mistake: When Gipsy Danger is lifted by the Kaiju in Hong Kong, they end up well above the clouds, and when Gipsy falls she eventually breaks through them. But in the shots when she is falling, you can see that there are no clouds around Hong Kong for miles. That, and Pentecost has a clear view of Gipsy through his binoculars.
Continuity mistake: During the birth of the baby Kaiju, when it faints Dr. Newton changes positions from farther away from the baby Kaiju to closer while he is curled up on the ground. The same thing happens the second time the baby Kaiju faints.
Continuity mistake: When Leatherback tosses Gipsy Danger onto the docks, she ends up on kneeling on her left knee. On the inside shots, we see Mako also on her left knee, but Raleigh is leaning on his right knee, meaning Gipsy Danger would be on both.
Continuity mistake: After in the battle for Hongkong Otachi spits acid against Cherno Alpha, the Jäger's head is almost completely eaten away. but when Leatherback emerges and joins the fight, Cherno's head is there again (before it gets ripped off).
Continuity mistake: The crew of Striker shoots Leatherback in the eye, but in the next scene, it's fine.
Continuity mistake: When Chuck Hansen is sitting at the dinner table arguing with Raleigh he is bareheaded. He stands up and in the next shot he has his baseball cap on. He had no time to put it on at all. It seems to have spontaneously appeared on his head.
Continuity mistake: When Raleigh is fighting co-pilot candidates, he strips one of his bo staff and somebody catches it. In the next shot it's on the ground.
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).
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.
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.
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.