Why aren't there more? Well, honestly, because they really aren't very useful.
I know that sounds absurd, but the truth is the idea is FAR better than the utility. Hey, cool, you could fire .38 S&W out of this! Will you, ever, even once? Probably not. Will you ever NEED that capacity? No, not in your lifetime, or many lifetimes.
Truth is, any extra expense incurred making a gun able to fire multiple calibers is wasted money if (...well,
because) such guns are already chambered in one (well, two) of the most common cartridges in the country -- .38Spc./.357Mag.
Even guns with only a 9mm conversion cylinder are very rare. 9mm is even more common, available, and cheap than .38 Spc., but any possible cost savings between the two will require tens of thousands of rounds to recoup the extra cost of the conversion parts.
Add to that whatever complexity is required to also handle .380 Auto, .38 S&W, and all those other "nearly 20 other rounds" (which I think might be hyperbole) -- NONE of which will even approach getting near to within sight of as common and inexpensive as .38 Special -- and the cost makes such a gun a self-indulgent novelty.
When folks cringe at paying more than $600-700 for a new S&W .357, why (except for the gun-geek novelty of it
) pay a lot more for a gun that can also fire a cartridge like .38 S&W that they've likely never ever even seen on a store shelf?
It's a bit like agreeing to pay $75,000 for a new Chevy pickup just because that one is designed to run on gasoline, diesel, kerosene, 104 octane race gas, and also coal oil, and spermaceti (whale oil). That's neat, but what good is it?