Because mobile dewars for liquid oxygen storage were spontaneously exploding. You need getter-free dewars for safe LOX storage on submarines, the task not easily done with the vacuum pumps and sealing technology of the era.
https://en.wikipedia.org/wiki/Oxyliquit
In particular, lead-tin solder was still not used, and sealing vacuum vessels with pure tin was common. This resulted in unreliable, leaky sealing, therefore getter to absorb leaked gas was needed (which would explode if exposed to oxygen leaked through another leaky seal). Welding quality control was also rudimentary, with rare and low-performance X-ray defectoscopes, and no ultrasound defectoscopes at all.