It's the most widely available for a very good reason, and that has to do with sound quality. Luthiers all over the world understand this and have had the opportunity to experiment with many different scale lengths.
Weren't you also earlier saying that any scale length works fine? Lol. Which again, I agree with. Long scale does work well I agree. Major manufacturers also like to stick with the traditional tried and true, which just happened to be what Leo Fender settled on. Still, as I said originally, most people say short scale has more fundamental. Look around online for yourself. Whether or not you agree is fine. I happen to agree with it, after extensive experience with many scale lengths of many makes. I don't want a super fundamental based sound anymore, hence me returning to long scale (plus other reasons).
Not all bass players are looking for all fundamental, by the way. If they were, flatwounds would still be the most popular strings. Most people want some harmonics/overtones.