They did their two biggest selling albums (behind Made In Heaven) and based on the sales of those, didn't think there was a big enough market for the rest, it's that simple.
If you can kind of force the market like Genesis did with their's, so that a surround mix is included with the definite issue, you get back most of the money invested in the studio time needed to do the mixes. Otherwise, you do a Pink Floyd and make a smaller percentage of your market, pay a bigger offset to get the surround mixes, or you simply don't bother with them at all. Queen chose the third option.
I believe a quad mix of the first album was done when they originally mixed it, so that one's been sat around nearly forty years, gathering dust.
What is left of your dream?
Just the words on your stone.
A man who learnt how to teach,
But forgot how to learn.