Because people are afraid of them therefore side pockets are a weakness to them. They do not understand that being able to pocket a ball equally well in any pocket allows for more shots options then always limiting your self to just certain pockets.
Who says they're afraid of them? I love side pockets. If the angle isn't too severe I find them to be softer than the corner pockets. They are like 10% bigger than the corners, you know, and with real shallow shelves.
But it ain't just about making the ball, it's about getting shape for the next shot. Making the ball in the side pocket can cause you to lose the CB on lots of shots. Where are you then?
I don't think the ease of pocketing a ball should be the main consideration in pattern selection. You should make the ball in the pocket that best enables you to get shape on the next ball. No use cinching a ball in the side if you lose the CB or leave yourself hooked on the next shot.