Militaries tend to be fairly conservative, so this is generally true of any given gun - civilian variants tend to acquire features faster. There's also the economic aspect of it - even small savings add up to a lot when you're ordering the product by the million, so there's a tendency to skimp on such small stuff.
And then, of course, there's the military bureaucracy. Here are some entertaining stories from David A. Lutz, who was the project manager for M16A2 at USMC:
"Ever wonder why Safe, Semi, and Burst are marked on the starboard side of the receiver? Well as the list of improvements increased to a point some 2 1/2 years into the program spilled over onto a second viewgraph, the last thing on the first page was the starboard side marking, the first thing on the second slide was a "mirror image" selector that pulged-in from the right side for us left-handed Marines. One Colonel who shall remain un-named insisted that I "lose everything on the second slide.""
Or here's the story of the M16A2 barrel profile change:
And then, of course, there's the military bureaucracy. Here are some entertaining stories from David A. Lutz, who was the project manager for M16A2 at USMC:
https://web.archive.org/web/20200708173852/https://www.ar15....
"Ever wonder why Safe, Semi, and Burst are marked on the starboard side of the receiver? Well as the list of improvements increased to a point some 2 1/2 years into the program spilled over onto a second viewgraph, the last thing on the first page was the starboard side marking, the first thing on the second slide was a "mirror image" selector that pulged-in from the right side for us left-handed Marines. One Colonel who shall remain un-named insisted that I "lose everything on the second slide.""
Or here's the story of the M16A2 barrel profile change:
https://web.archive.org/web/20200708174036/https://www.ar15....