They never took off, and to be honest they were never really that useful for the end user either - they offered very little over a normal CD, and both the media and the equipment needed to play them was much more expensive (as well as having a much more limited selection.)
The one capacity in which they were useful was recording, as they were much nicer than tape in this regard - in the early 2000's we recorded a number of tracks using MD for this reason (of course, CD is completely unsuitable for recording as you go, and this was in the days before you could rely on your average computer to record large amounts of audio without breaking somehow.)
However, with the advent of SD card recorders, good free programs like Audacity and the price of good recording interfaces freefalling, there's really not been much case for them for a long time now.