The Japanese cameras beat Leica to the starting gates; less expensive and perfectly adequate for the job. Better distribution too, they were everywhere, and Leica's were more limited to boutique shops. Remember, this was the days before the internet. Plus, the newspapers largely adopted Nikon, for many reasons, some of which had nothing to do with quality (e.g., price, availability lenses, ect). The Nikon F was such a reliable tour de force; why change unless something was demonstrably better. Leica R lenses are better (to my mind), especially at the wider apertures, but that doesn't mean, for example, Nikkors are bad. In fact, they were and are good lenses, and perfectly adequate at getting the job done and less expense. Nikons were expensive back then too, and Leica's were even more so.
Having said all that, my favorite mechanical SLR is the Leicaflex SL. A big, bold, beautiful viewfinder where the subject just pops into focus in a dramatic way. And what incredible damping with a special breaking system. I can shoot a Leicaflex one or two stops slower then a Nikon! In my experience, however, of five to seven years of sole use, not as reliable as Nikons (eg., F, F2, F3), with heavy repair costs, because a lot of dismantling needs to take place. Nikon F has a strong reputation of reliability and ease of repair. Still, a Leicaflex SL with a 90mm is just one of those things that need to be experienced.
Then the R line came along, and they were pretty much comparable to Japanese cameras. Not worse, not better, just pretty much the same as Minolta, Canon, Nikon ect. The R8/R9 -- great ergonomics and viewfinder -- just arrived too late.
Good pics can be taken with any gear. Leica SLR's (the R line) are generally just a good as their Japanese counterparts but not better. The real question should be why is Leica M stuff so damn expensive ?????? You can get that rangefinder experience with a Canon or Zeiss Contax at just a fraction (maybe 10%) of the cost. Still, as we all know, ha, ha, serious photographers use, I mean, NEED, a Leica M!