Despite those who would publish charts and graphs to illustrate why "this" or "that" is superior, photography is a black art in which there are no absolutes. Photography is merely the pursuit of the permanent capture of light, and every individual who practices photography has the ability to do it in any manner they see fit, if they know how to do that.
Practitioners of this black art eventually, after the basics have been mastered, find nuances to be exploited.
There are products in the world that do the job; then there are products that allow for nuanced interpretations with little or no muss or fuss. Leica falls into that latter category, and that's the essence of why those who own them are so fanatical about them.
The rest of the manufacturers offer cameras with automatic features that must be defeated, programmed, tweaked, or personalized. Leica allows you to quickly make images as you see them without having to fight with the camera... again, provided that you understand the mysterious and nuanced workings of this black art.
The reason pros moved on from Leica thirty or forty years ago was advertising... and customer service. Leica was all but dead. Canon had pro services for their cameras that were unbeatable in the '90s. And the last holdouts (me) moved over to Japanese DSLRs in the mid 2000s when there were no Leica digital bodies on the horizon and I needed digital to stay competitive.
There are some of us "pros" who are back in the fold, but most are too heavily invested in DSLRs of the Canikon variety to unload their equipment and move back. I was fed up with automation and fortunate in that I was in a position to move back to Leica.
Shooting a coincident rangefinder camera is a different experience from shooting an SLR and it's very difficult for some photographers to make the mental leap required to use a rangefinder camera to its fullest. For me it's easy because I started on a Leica forty years ago, and then had to make that mental leap from shooting with a rangefinder camera to shooting SLRs early on. And I've never been as comfortable shooting an SLR as I am a coincident rangefinder camera. For folks used to significant levels of automation, shooting a manual camera (especially a manual coincident rangefinder camera) requires them to actually learn the properties of light, plane of focus, DOF, and manual exposure, and then make their own decisions about the best way to capture the image, and then transfer that knowled to the camera settings. In other words, moving to a manual rangefinder camera requires the owner to actually learn the basics of photography and some folks just aren't up for that either.
There are a million shades of gray in every facet of this black art we call photography, and Leica still allows the photographer explore all of them without being trapped into just capturing the few shades of gray that the programmers thought you'd want to capture from the relative safety of a single perspective with a long zoom. Shooting with a Leica coincident rangefinder body and primes means moving to explore different perspectives. It's a very different working style that develops.
The Leica "mystique" (at least in the M line) assumes that, in order to make useable images, that the a user is photographer. That's what makes Leica unique among today's camera offerings.