Because in some ways they are unbeatable. If you want to be discreet a little cunning allows you to sling the body under your armpit and hide the lens in your hand. Not me guv. The shutter is so quiet you can photograph people without disturbing their concentration. Or being noticed.
Poor light is no restriction.
As there is no mirror to flap up and down the only camera shake you have is all your own. Rangefinder focusing is slow compared with a SLR but this can introduce a different, more thoughtful pace to the proceedings. Leicas are not to be hurried, nor are their pictures. Perhaps it is because they are so cranky that once you get used to the Leica they do cast an powerful spell. I find the sound and feel a welcome change from the zip-zip of the modern motor drive.