I suppose it was missing as it was never really a music distribution format. More for home recording.
I remember doing a project on MiniDiscs in year 11, thinking they were going to be huge.
...and then there was Video 8 (even Wikipedia seems to have forgotten that at one stage this was touted as a home video and audio format rather than just for cameras: a friend had a home Video 8 player and one of its party tricks was to digitally record several CDs on a tape) and DAT (which took off as a pro recording format, but was probably priced-out of the domestic market because - shock, horror - you could use it to make an exact clone of a CD with no loss of quality!) and even Digital Compact Cassette (backwards compatibility with analogue cassettes!)
I think the problems were that (a) MiniDisc and DCC were knobbled to stop you cloning CDs (original MiniDiscs couldn't hold a whole CD without compression) - the industry was stuck because they couldn't accept that that was what most people wanted recordable media for.
(b) Sony, in particular, were in competition with themselves (MiniDisc vs. DAT - also MiniDisc data vs. the floppy and Sony's HiFiD high-capacity floppies) and
(c) the humble cassette tape was really cheap, got the job done, you couldn't really hear the difference between a CD and a C90 if you were in a car, on a train, or walking beside the road, people had cassette players everywhere, and pre-recorded cassettes were (for a period) the biggest-selling music format (see the original poster's graph) and people had a shedload of tapes.
Of course, turns out that by the late 90s, if you wanted to clone a CD with no quality loss you could use... a CD! - but what people really wanted was to compress the hell out of the CDs so they could fit dozens of them in their pocket - which ISTR you could do with MiniDisc, but Sony daren't advertise that as a Selling Point.
In the UK it is (or was until recently - ISTR it was going to be changed a few years ago but got bogged down, but I haven't been following it) technically illegal to copy recordings even from pre-recorded media you "owned" for your own use, although nobody has ever, or is ever likely to be, prosecuted for making a mixtape (unless they run off 1000 copies and flog them). Never stopped Amstrad making a mint selling dual cassette players, or the Virgin Megastore happily selling you the latest CD and a six-pack of C90s - but it was a bit "don't ask - don't tell". If you want to make a device who's Unique Selling Point is the ability to copy CDs you have to be a bit cagey about it or the advert is likely to be banned.