Probably hubris.

"Just because something failed every time everyone else tried it, that doesn't mean it'll necessarily fail for me, because I know where they made their mistakes and fix them!"

It depends on which 9mm you mean, and also what kind of gun. "Nine millimeter" by itself just refers to the projectile diameter, and there are a lot of 9mm cartridges. Some of them work just fine in blowback handguns. .380 ACP, for instance, is known in the metric world as 9×17mm Browning Short, and it's used pretty commonly in blowback pistols; so is 9×18mm Makarov, although, thanks to the peculiar way the Russians measure their cartridges, that's actually a 9.3mm projectile, and 9mm Mak is riiiight on the edge of usability in a handgun action. 9×19mm Parabellum, by far the most popular 9mm cartridge, can be and is widely used in blowback submachine guns and pistol-caliber carbines, because their moving parts can have a lot more mass to damp the recoil with, but every attempt that I know of to make a blowback 9mm Para pistol has failed, because the cartridge is too powerful for that.

9mm Largo is a bit of an oddity, in that it was only ever really used by Spain and is obsolete even there, so you don't see it around much. It's 23 mm long, four longer than 9mm Parabellum, but not as powerful, because it's loaded to a lower pressure. Only slightly lower, though. It's still too powerful to be suitable for use in an unlocked handgun, to my eye, because in order to be safe—and the Astra 400 is safe, I should stress that—the gun has to be too heavy, and have too strong a recoil spring, to be comfortable to use. It works, and it's not going to blow up and put your eye out,* but a comparably powerful locked-breech handgun would be smaller, lighter, and easier to use.

On the other hand, blowback actions are a lot simpler to develop and manufacture, and thus cheaper. It appears that in the Astra's case, the Spanish Army of 1921 decided that saddling its soldiers with a big, heavy sidearm which you need Conan arms and heavy calluses to operate was worth the cost savings of not bothering with a locking design. Handguns are very rarely militarily significant, anyway, and by 1921 most European armies had figured that out.

It is an imposing gun, I will say that. Its size alone makes it look much more formidable than it really is.

--G.
* unless there's something wrong with it, of course, but that's a contingency that wouldn't be the design's fault
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