Part of the reason may be that it people cant figure out what it does especially better than other more common loads and guns. Id be one of them.
Those that have mentioned using them (not in the recreational sense) haven't been all that impressed with its performance overall.
It's an insanely light weapon, especially loaded, especially when you are carrying multiple magazines. Feels like a damn toy; even airsoft is generally heavier due to the pump mechanism. The loaded gun is lighter than an empty G17.
Recoil is about 1/3-2/3 that of 9mm. The gun also has a large grip and aggressive checkering; makes rapid fire highly controllable.
The round is also fast enough and of a high enough BC (ballistic constant) to be noticeably flatter shooting out to 100 yards.
The real problem with the operators out there who have been dissatisfied with both five-sevens (who actually adopted it?) and P90s is one of expectations and intended use. Remember, this gun was to be a PDW for rear-echelon NON-COMBAT guys, as was the pistol (which is used almost exclusively for mundane activities like guarding, if I am told correctly). It was never intended to be the tip of the spear for guys knowingly going into combat --they were supposed to be using the approximately 3X more powerful and 2X bulkier 5.56 weapons for those jobs, at a minimum.
So what is the first thing hot-shot cops, operators, and other fools who fell for a slick marketing campaign do? Press the ultra-light carbine and pistol into aggressive/offensive roles where their performance and power are far more critical factors than their light load out weight, simplicity, cleanliness, or capacity --then declare them abject failures. I have to wonder if the strengths of the platforms might have been appreciated more by a group that has to actually carry and live with the things day-in-day-out, vs. operators who get to ride a transport all the way from the armory to the objective for a short/fast mission. Law enforcement especially.
You know, the AR15 makes for a lousy concealed carry pistol; must mean the round is far too powerful and the gun far too heavy/bulky/dirty for anything, and therefore an answer to a question nobody asked. What's that? Pistols make more sense than rifles for daily carry because they're smaller, lighter, and weak enough to be fired without shoulder support effectively? But if you're going into a fight you want the most effective weapon, right, so why drop to a weeny 9mm over 5.56?
Same reason the P90 was developed to replace the (then) M16A3(?) and the five-seven to replace the M9; when the soldier's job isn't to close with and defeat the enemy (which it usually isn't without a fair bit of warning and preparation, given modern tactics) there's a huge advantage in getting their weapon lighter, smaller, and a smaller hindrance to their actual jobs (while still being present to heed the call in an emergency). The M4 more or less splits the difference as far as the rifle/PDW was concerned, and more importantly allowed Uncle Sam to avoid having to make an actual decision since it was basically the same rifle they started out with
TCB