Pike was an extremely well regarded captain (DIS Choose Your Pain) in a crucial period of Federation expansion and solidification, many years before he suffered a horrific fate saving a group of cadets. Plus whatever events of significance he may be involved in during SNW's run. I suspect the accident, heroics, and gruesome injuries beyond the capabilities of Federation medicine made for a pretty big story, elevating Pike's fame in the eyes of civilians not already familiar with his very impressive service record. Making his birthday a holiday is a believable honor for someone now widely regarded as a hero who essentially made the ultimate sacrifice, but is technically still around to appreciate the gesture.