It started with their ability to perform under adverse conditions. That's why the military adopted them.
While true for a number of guns, it didn't start out that way for the AR. Gen Curtis Lemay was looking for a replacement for the M1 carbine his Air Force SPs were using. At that time, the Air Force was still getting all its small arms, and support (parts, etc.) from the Army. And the Army was dropping the M1 Carbine.
So, faced with the choice of using the carbines until they ran out of parts or looking for another rifle, Lemay went looking, and (according to stories) was shown the AR rifle at a party. Apparently he thought it would be a fine choice for airbase security. (not down in the mud infantry combat, which the Air Force didn't do) Also have heard that JFK was taken with it, and enjoyed shooting it.
It was the "brilliant" bean counters in the MacNamara Defense Dept that forced it on us as a general infantry rifle, the M16. There's a lot more to the story, but that's the basics of its beginnings.
As to a pump or straight pull AR, to get around the legal restrictions in some places, ok. Just understand that as long as the rest of the nation remains relatively free, the market will be small.
AND, there is a potential unintended consequence if you go that route. Right now, the gun bans all are worded to include semi-auto AND magazine capacity (often phrased as "the ability to accept magazines over XX round capacity, or something similar).
What they are afraid of is a large capacity firearm that can be fired quickly. Once they realize that you are flouting their rules by using a manually operated action, they will simply reword the laws to include ALL firearms that can accept a ....rather than just semis...