I believe its supply & demand. There are alot less MFSL than the other and is US only release. The blue box stereo was released in many countries and for the most part sound similar. Now look at the limited mono released box sets both black & red only released in the UK & Japan, MFSL box set is cheap compared to them. Open box set MFSL $500 to $900. Limited open mono from 1200.00 to 2000.00 to start the bidding. I have seen sealed box sets by MFSL but never a sealed red or black box set. IT's all supply demand even in the used department.
MF Beatle Boxes are awfully dear.
Back in the 80's, I was looking for a box set of Beatle records, and a set of Japanese pressings on high quality vinyl sounded better, to me, than the MF, which seemed to have exagerated bass (not so unusual for MF records) and not so much clarity in the highs. Several reviewers shared that view. Older mono Parlophones were considered by many to be ultimate versions of the genre (the artificial stereo versions of the early records can be irritating - like hearing George play in one channel while singing in the other). Now, the MF sets seemingly command the highest prices. Did the "conventional wisdom" somehow change, over time, did everyone just "forget", a younger generation just doesn't know any better, some strange artifact of marketing hype, or what?