I'm not sure there's really one answer. It is a more limited pressing than the stereo cds, and it is also a "better" box, IMO. The mono's mini-lp sleeves are much nicer than the digipaks on the stereo discs. The discs have individual plastic sleeves, and cardboard sleeves, as well as a plastic sleeve for the lp sleeve as well. Plus the mono box has a separate booklet, whereas the stereo box doesn't (but it does have individual booklets for those).
I don't think it has to do with the mastering being any more expensive, though I could be wrong.