Star Trek: 10 Things You Didn't Know About Captain Pike
5. Why He Missed The Klingon War
When the Klingon War first began in 2256 (two years after The Cage), Pike was leading one of the Enterprise's five-year long deep space exploration missions.
During the war, Starfleet ordered the Enterprise crew to continue their mission, which Pike was not too pleased about. He and the rest of the crew wanted to help defend the Federation, but command chose to keep them away to use as a last resort. They didn't want to risk losing some of their most talented officers until they had no choice, but Discovery was fortunately able to end the conflict before the Enterprise needed to get involved.
The Enterprise was also pretty far from Federation space at the time, seeing as the point of their five-year-mission was to chart unexplored space. Even if they were ordered to help fight in the war, their journey would take months, if not years.