Because there is so much conjecture over what is a "good" Marlin and what is a "bad" one. Which is kind of the same problem with Winchesters themselves (is there such a thing as a pre 1964 1873 Trapper in .357mag? Even if there is, certainly it won't be CCH finished.)
Also 99.99% of the time, when I find a Marlin (good or bad) its either in .30-30 or .45-70.