I don't know why nearly all cameras have the shutter release on top, to be pressed downwards.

But I do know that a shutter release on the front of the camera, to be pressed towards the back, is not much, if any, better. With my Sawyer's Mk. IV (rebadged Primo Jr.), pressing the shutter release tends to rotate the camera around its vertical axis. Keeping it steady and pointing where intended requires some effort.