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Constant
A very good question, but one that has a wide ranging answer. The obvious answer is that a low f ratio telescope is not necessarily "better" any more than a fast lens on a camera is automatically better. It depends what it is being used for.
Secondly, just like camera lenses, fast scopes are more expensive, often prohibitively so, so for makers to focus on making fast scopes would mean that they would cater to a small market segments. Again relating back to the photographic community, there are a lot more bog standard kit lenses sold that fast telephotos!
Thirdly, until fairly recently, say the early 90s, most scopes were sold for visual use. While that is probably still the case, the market for dedicated imaging scopes would be much larger now than it was 20 or 30 years ago. This is as a result of the availability of CCDs and DSLRs, computer processing etc rather than film. For visual, the main impact of f ratio is on the size of the scope. The popularity of SCTs since the late 60s attests to the value of having a decent aperture in a compact package.
Cheers
Malcolm
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