when we start talking about reduced-power magnum loads or specials, I often start to wonder why people don't just break ranks and bump up to .45 Colt in revolvers or .45acp in semi.
When you are talking only power levels one does wonder why, but the real world reasons can be many, and varied. One reason is simply that some folks don't have (or want) multiple handguns. Some might settle on the .44 Mag as a good, or good enough "do it all" round. And it can be, provided you know enough it to be able to use the ammo best suited to the task.
If you are a revolver only guy (and I admit they are becoming rare), the .45acp in an auto pistol, no matter how good, just isn't for you.
Also, that same revolver guy gets nothing, in terms of gun advantage from going to a .45 Colt or .45ACP in a revolver, as these guns are the same size as the .44s. So for a carry gun, its essentially the same package.
.44 Special, lighter loads in .44 mag brass, regular .45 Colt, and .45ACP are all very similar in self defense performance. All use large frame revolvers, however, only the .44 Mag allow for the use of magnum level loads, when/if desired.
Several have said .44 Special is better than .44 Magnum for defense, and to a point they are correct, but only when comparing SPL to Magnum at full power. If they are saying the .44 Special gun is better than the .44 Mag gun, that's just silly. I, personally don't have any interest in .44 special handguns, I have .44 Magnums. I can shoot specials if I want, but the guy with a .44 SPL handgun can only shoot .44 SPL. I would choose the magnum gun, even in a snubnose, because of the ability to shoot magnums, when desired. Would I load it with magnums for defense? No.
I have pistols in .44 Mag, .44AMP, .45ACP, .45 Colt, & .45 WinMag. I know a few things about these rounds (and the .44 SPL as well), and I can tell you, from personal experience that when you are talking about the things usually considered important for a defensive pistol cartridge (a given minimum power level, and controllability for rapid follow up shots, the magnum rounds, loaded as magnum rounds, are a poor choice for most people.
Guns in magnum calibers, loaded with less than magnum level ammo can be as good as anything. (leaving aside the revolver vs auto round capacity debate).