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.
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.
This matches my experience on shared pedestrian / bicycles low density roads in countryside. 30 km/h (a little less than 20 mph) is the limit between okish and dangerous. Actually pedestrians start to worry a little before that speed but of course half of them don't see the bicycle approaching. The cyclist see them a long time before overtaking and assesses whether they are safe to pass or require a slow down.
This is a good example of how we get bad laws... Should overweight cyclists be forced to have a lower speed? I can still get my ebike to 25 mph down hill, I would never dream of passing a pedestrian at that speed. I picked 24.5 for a reason and not 25 BTW.
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.