I can't speak for everyone, but there is a spot in my city where I will briefly get up on the sidewalk with my bike, and for me the reason is always the same, the street isn't safe. The downtown where I work has no bike lanes, no shoulder, and people blatantly running red lights, blowing stop signs across 4-6 lanes of traffic while speeding. There is no greater joy than getting doored because the other alternative is swerving further into the road and getting run over at 40+ mph. For those two blocks it's safer for me to slowly push the bike along with my feet until I get to the next block and ride in the street again. Too many cyclists that get on the sidewalks don't show deference to people walking, the same way they wished cars would show some deference to them.