Simple: "CaUsE It'S LeIcA".So why do people pay the high prices for the Leica lenses to convert to Nikon when they could just buy the Nikon lens in same focal/aperture and save the money for film?
Not everyone, but plenty of people go for Leica just because of the name. Owning Nikon is like owning a very well-working and reliable Japanese watch, which however, is unremarkable and hardly anyone notices it on someone's wrist. Leica on the other hand is something well-known and usually associated with quality. A head-turner, jaw-dropper and in case of R8/R9 also a nut-cracker.
The rest of the people - I don't know. Even I converted my Olympus (yes, olympus, not even Leica) 100 mm lens to Nikon F mount just because I like the look of that lens and didn't want to part with it when I moved to digital. So who am I to judge, right?