Maybe we find out why things work in mice and not us.

This works in mice with small tumors for two weeks until the experiment ends. It's quite different form working in humans with big tumors for 5 years.

Mice are good for early tries. The researchers had 9 bacterias and only 1 was successful. Experiments in mice are cheaper and have less ethical problems than experiments in humans.

(Hey! They even injected the cancer cells in mice and waited a week until it grow. Nobody will approve that in humans.)