I voted "because I wanted one", but would make the additional caveat that I found myself in the somewhat surprising circumstance that I could afford one.
As to why I wanted one, well, it's all about the rangefinder (or, to be more precise, "viewfinder combined with a lens-coupled split-image and double image rangefinder with illuminated, parallax-corrected, brightline framelines"). I find I can't get along with EVFs, though in some ways I wish I could. I suspect my aging eyesight has something to do with that but I also perhaps rationalise that I feel a difference between seeing the scene itself versus watching it on a TV screen between me and the scene (all EVFs I've tried feel that way to me).
Beyond that, the only ways I know of to have visual/mechanical confirmation that the camera is focused on what I think/want it to be focused on, with an OVF, are through confirmation of focus on a properly adjusted screen (eg. a single or twin lens reflex system) or optical rangefinder. I know, through use of cameras like the Hexar AF, Contax G and T series and even the OVF of an X Pro-1 that a nice light indicating focus lock may mean the thing is focused, but doesn't really tell you what it's focused on. Call me insecure, but I like to confirm that focus is where I want it, rather than on something else the focus system may have latched on to.
In addition, I see and compose differently when using a camera with a finder window rather than a reflex system - both through seeing outside the framelines and also because a reflex system shows you what's in focus at widest aperture, leaving you to imagine what comes into focus as you stop down, while a viewfinder system shows you everything in focus leaving you to imagine what you're subtracting as you open the aperture. I find I do things differently with the different finder systems - not better or worse, just differently.
(EVFs no doubt give you the possibility of seeing things as they would be at the taking aperture, without overly dimming the finder, but I still don't get along with them.)
For a number of reasons, I wanted to do this with digital as well as with film and I wanted to do this with my existing M and LTM lenses at their existing (for film) fields of view - for that Leica was the only game in town. When I could afford to buy one I did, and I've been glad that I did.
...Mike