Conductors tend to follow a certain interpretation of a composer's music. If I like the way Leonard Bernstein or Michael Tilson Thomas conducts a Mahler symphony, I'm likely to want all of Mahler's symphonies as recorded by one or the other (or, heaven help me, both), not one conducted by Leonard Slatkin (tho he's really good as well) and another one from Fritz Reiner.

Though I'd make an exception if I could find the Mahler's 2nd conducted by Eugene Ormandy I grew up with in the late 1960s, though...