“Neither good nor bad books are salable.” Given this sanguine assessment of market forces, why doesn't the press simply go out of business?

A friend, editor at another major university press, writes to clarify:

“It can be difficult to predict when a book might sell (say) 900 copies, and when it's likely to sell only (say) 350 copies, but university press editors spend a lot of time doing just that. The difference is usually not a matter of quality in any pure sense. Rather, the 'economic argument' in this typical example (the 350 copies that we can't afford to publish vs. the 900 copies that we can) usually has to do with the author's aims, the traction they are likely to have within the author's field, the vitality of that field, and so on. It's on this level that the economic argument takes place, and quality alone doesn't clinch it. At the same time, a badly done book won't sell even if it speaks directly to the scholarly interests of a large audience. That's what your editor's comment means (at least to me), and it shouldn't be controversial.

The really hard question is what are the qualities that allow a book to speak to so modestly 'broad' an audience as to justify a print run of 900 copies?”