Because kinetic energy is proportional to the square of velocity, and 25mph is 56% more energy which is a huge problem in a collision with a pedestrian or a crash.

https://usa.streetsblog.org/2016/05/31/3-graphs-that-explain...

It's also a big issue with response time and with the rate pedestrians expect bicycles to be travelling. You shouldn't bike on the sidewalk because no one is looking for vehicles going more than a few miles per hour (walking). You shouldn't bike more than 15mph on trails/lanes shared with pedestrians for similar reasons. 20mph is a tolerable compromise, 25mph is flat out dangerous.