In the movie Pacific Rim the robots kept losing and almost at the last minute they remembered that they had a super powerful sword and cut the kaiju in half. Why didn't they use that in the beginning of the movie?
You would have to look at this from the point of view of the filmmaker as well as from the point of view of the actual character
From a filmmaker's perspective you would want to keep your 'best-for-last' moment to be as surprising and awe-inspiring as possible. Revealing the sword earlier to the audience would spoil the climax.
As far as the characters are concerned one of the jaeger pilots Raliegh Beckett wasn't aware of the sword in his Jaeger in the first place. It can be argued that Mako Mori didn't remember this detail since she was a novice pilot. (I know this is far-fetched but I am giving them the benefit of doubt.)