A lot of it has to do with barrel length too. Try finding a 2-inch,or less-9mm.
Also the cartidges they are loaded in have different volumes of powder charge that can be loaded,and different amounts of pressure they can handle-1 vs. the other.
Neither of the rounds are ideal for defense-despite the hype of "modern" loads for them. Especially in shorter-barrel pistols/revolvers. They can do in a pinch,and sometimes you might luck out.
But-popular they may be with civies-there's very valid reasons all the PDs abandoned .38s ages ago, and went with 9s. And now there's very valid reasons nearly all of the PD's (and the U.S. Military)are running as fast and far as they can from 9's-and trying to move up to .40s/.357sigs (police) and .45s. (military) Because they just dont cut it when you need it to.
But they are handy as being fairly small/light/med. bore guns.There's that advantage-but at the compromise of the bad guy saying "err..what did u just throw at me" and the BG saying "...." well, nothing,because he's either dead or well on the way to being so.