IMHO what really killed the QL was two factors:
- the microdrive: considered unreliable. Even if they were not, would you trust important data to a very very thin tape speeding in and out of a little black plastic box, controlled by Sinclair quality engineering, always on the verge of turning itself into one thousand Moebius loops ? Anyway, that was the impression it gave.
- Software. In those days of darkness and home computer, you could not really count on compatible (or even semi-compatible) software: each home computer came with its own complement of software (speak me not of MSX).
This depended on the computer maker shipping documentation and prototypes to software houses which had to really believe in the concept, and sink money in it.
This chemistry apparently failed to work for the QL. At least in Italy for long long months there were about 2 pieces of software available for it.
Later on some SW appeared, but it was too late: the sales had lost momentum.
The QL failure was a pity because the machine had a very good keyboard (full size, QWERTY, reasonable key travel), good video (80 columns !) and acceptable bundled software.
And the 68000 (even in its cheap version, the 68008) was a good CPU after all.