I already have an AR15, so some of this may be moot. Still…
I bought an AR15 recently. Lots of debate as to the reliability, function, design, etc, etc, etc and whether there is better out there. I have been following some of the threads, most notably U.S. Army Agrees That The M-4 Stinks. Many have suggested better combat rifles than the AR15. Some suggest the AK, some suggest the HK416, some suggest the gas piston Ruger, some are waiting for the Remington ACR, etc, etc, etc.
When I bought my combat rifle, I decided to go with the AR15 platform because as of right now, it is the military’s main weapon. It is ubiquitous and parts are widely available. As are accessories and extra magazines. And finally, it was substantially less expensive than a piston based rifle. I.e., $1200 for the AR15 vs. $3500 for the HK416. The parts and all the accessories are also cheaper than the alternative. For example, magazines can be purchased for about $15 each where as magazines for the HK416 are like $75 (don’t quote me. But they are substantially more expensive) and they are harder to get. Parts for the HK416 are also harder to get as well as more expensive.
So I guess it truly comes down to dollars and availability. The AR platform is much less expensive than the alternatives and easier to find parts and accessories for. Why go with a platform that would be almost impossible to maintain should TSHTF and/or more expensive to maintain on a regular basis? In an urban conflict like we would most like see here in America, the gun should work just fine.