I'll give ya a very good reason right here -

What, nobody but me rides horses anymore?

The single action revolver has always been the sidearm of choice among horsemen. You can grab it out and have it in hand, but it is still perfectly safe, no danger of shooting yerself or the dang blam horse if it does something unexpected. You cock back the hammer and take yer shot, and instantly the pistol is again rendered perfectly safe, so when the horse goes nuts because you’ve just ruptured it’s poor ear drums with the noise of yer .44 magnum and throws you while yer still clutching that hogleg to hand, you don’t shoot yerself ( although you may wish to shoot the horse at that point – ask me how I know )
I wouldn’t dare use any other handgun on horseback, even a double action revolver. The thumbuster is just plane safer.
And I don't think a single action gives up anything as a defensive arm to the newfangled and more complicated just-yank-the-trigger kinda revolvers, or them fancy complicated reciprocating guns that jam all the time and when they do work they throw yer brass everywhere.
The single action encourages aimed deliberate fire. Misses don’t win gunfights, only solid hits do. And a fella can accomplish that aimed and deliberate fire just as fast with a six-gun as with anything else, lest wise if we’re talking about real guns in manly calibers that recoil. By the time I come down outta recoil and am putting that front sight back on target, my six-shooter is cocked again. I can’t shoot a double action .357 with powerful loads – and hit anything – any faster.
Now is when folks is gonna start to moan and Rosie O’Donnell about how durn long it takes to le-load a six-gun.
Tell me, when was the last time any of y’all emptied yer gun and needed to reload it during a fight?
Maybe I’ve lead a sheltered life, but I’ve never had to. ‘Been a few times I was right glad to be armed, but so far I haven’t had any major firefights. Five or six powerful and well-aimed shots seems to be enough, for me, anyway. I also don’t worry about carrying around enough ammo to refight the battle at Adobe Walls.
Today at work I’m carrying a .44 special thumbuster and no reloads. Scary I know, but I reckon I’ll make it through the day.
Six-shooters are also the most rugged, simple and dependable of sidearms there is to be had.
OK, I’ll allow as how those Ruger just-yank-the-trigger-to-shoot guns is also pretty durn rugged. A feller can even dismantle one of these easily enough, unlike a Smith or a Taurus or Colt, or any other yank-em-to-shoot guns.
But if it was you in a canoe on a wilderness trip where you figured you stood a reasonable chance of taking an unintended swim, what sidearm would you strap on ( ifin’ you was stupid enough to strap on three pounds of steel while in a canoe. Not the easiest things to swim with – Ask me how I know ).
Wanna take a Smith swimming with you? Gonna take off the side plates to dry out the guts or just hope they don’t rust up?
Same goes for reciprocators, lots of little bits to rust up inside (although I gotta allow as how those Glock things are right durable. For reciprocators, anyway. )
I’ve manage to jam a single action gun only once in my life, when it got a wood chip wedged up under the hammer. I kid you not, I was running a chain saw day after day cutting my winters firewood while wearing a thumbuster in an open top holster, and somehow a chip just the right size got under there. Every other type of gun I’ve owned – and I’ve had double action revolvers, H&Ks, 1911s, an XD-40, and a big pile of odds and ends when I was single and still on the Gun Of The Week Program, and they are OK, some even pretty durn good, and I still own three Glocks because they ain’t to accurate but they almost never goof up, but nothing compares to the, simplicity, strength, accuracy and power of a Ruger thumbuster.
( Gosh Ruger should pay me for this! ).
So if yer an outdoorsman that does more than shoot his toys on a range now and then, you need a sturdy thumbuster, and Ruger makes the best of those there ever was.
Now how about all you survivalist types sitting atop a bunker of Spam and Sharman waiting for the world to end? Y'all need a single action too.
I think Ruger six-gun is the last of the sidearms likely to still be popping off long after everything else has fallen silent.
As has been mentioned, just about anything that can be shoved into the cylinder can be fired. I’ve made .44 Extra Short brass for my .44s by taking up .45 ACP brass I found in the woods, and re-sized it all the way down to .44. Ya don’t re-size the rim of course, so you wind up with a very short .44 case with just enough rim to hold it where it needs to be. Work fine.
Talk about not being ammo sensitive!