Because caseless rounds have a severe problem with removing heat. Cased rounds dump most of the heat into the shell, which is promptly physically ejected. Caseless rounds dump that heat into the chamber, where it disperses slowly. While you can fire mag after mag for hundreds of rounds from a regular design, a caseless system will overheat way too early. H&K had such a design long ago, but they never resolved the heat problem.
If I recall the history correctly, they then had what is called a "cook off" problem. Troops really don't like this problem.
It’s only an example, and it may be a technical issue that can be overcome. Who knows? But right now, all were looking for a commercially off the shelf solution.
The problem is simple. All the present day alternatives aren’t really worth the trouble, cost, and risk. None really give a real “break through” in performance or capabilities like the M1 or M16 did when they were new. Weapons that lasted for a long time and gave a serious advantage to our troops in some way in their time, they all had one thing in common, they were more revolutionary in concept or design with new features or vast performance advantages. Perfect example of the opposite was the M14, which was never an awe-inspiring weapon even though it was reliable, durable, and had lots of firepower. It was an evolutionary development based on the M1, and outside some special applications where it does well, even today BTW (Sniper rifle in some variants), within three years of its implementation it was being replaced by the M16 and you had commanders in Vietnam begging to get that thing replaced which finally happened by 1965.
What we have today as options, are off the shelf designs in part not much younger than the M16 itself. The M-8 is essentially an AR-18 (2 years younger than an M16 design), the SCAR which some will refute, isn’t new either from how it operates. It’s all old potatoes reheated and repackaged, using of course tan plastic instead of green! (The important details)
What these discussion deteriorate into amounts to squabbling over whats better, the Springfield XD, or a Glock!? Then someone will post a picture that looks cool, and the cool picture wins. I personally feel the Springfield has an advantage, does it mean much though? No. Is replacing a Beretta with a USP really going to improve anything? No. And there were voices screaming for that nonsense years ago as well. Its the first guy with a mini-ball, the first rifled barrels, the first rifle or revolver to shoot a cartridge vs. being front loading .. those are real advantages.
The goal isn’t to pack useless technology into a design, or over-engineer something that works well as it is, but to employ and identify new technologies in a way to give more than a “marginal” gain in performance or add some new capability all together. What we need today is someone like DARPA in conjunction with Armalite (Just an ***example*** again, because that was the case with the M16) to develop a new weapon. We have come a long way when it comes to computing power, optics, material sciences, propellants, even alloys, power supplies...... The real achievements (and they have been largely capitalized on) are in the realm of optics, pointers/illuminators etc . Some of the stuff available today is truly awesome, but the basic gun is bla.
Start bottom up; analyze the needs, the environment, the Soldier/Marine, the threat, the new materials, optics, propellants, and electronics out there, bounce that off of what is mass producible, logistically sustainable, and from a cost realistic, and develop something that has future growth capabilities designed into it. Design a gun incorporating the new technologies and around the needs; not modify some old crap, giving it a cool name, and peddling it off as something new and great. Maybe that abortion of an OICW killed this idea, and today were looking for a quick, lower risk, and cheap off the shelf answer, I dont know. But were settling for less than what we could have, and that I do know.
The Infantry museum use to have a nice display of the prototypes of the M16 and its evolution:
I dont see it anymore, and thats sad. But the point is that they developed a weapon based on the needs and data gained from real analysis, and incorporated new concepts (high velocity, small caliber, unstable round at impact) that allowed the weapon to be lighter, more compact, and carry more ammunition at the same weight while retaining the same if not having greater effect on a human target. They used state of the art materials (plastic and cast aluminum at the time the materials were wood and machined or stamped steel) to make the weapon less susceptible to rust and corrosion and make it lighter yet. What they ended up with, was something that was truly awesome; a gun lighter, more compact, with greater accuracy, easy to learn to use, high resistance to rust and corrosion, manageable on auto, with more ammunition at the same weight, that had in most cases more effect on a human target It was done right, and 43 YEARS later this weapon is still around.