There are (or at least were) several requirements of a military rifle that were/are not met by most sporters. The first was ruggedness; most sporters just would not take the abuse dished out in the military, mainly because the stocks were too light.
The second was that the average GI be able to understand and maintain his rifle well enough to keep it functioning. That meant easy disassembly into major groups, few small parts to lose, and the ability to clean out dirt, mud and general debris. Ideally, all disassembly would be accomplished with a simple tool or with parts of the rifle itself. (Whether the M16 meets this requirement is open to question.)
The third (and increasingly less important) is the ability to take a bayonet. One of the main objections to the rifles used in the Revolution was the lack of a bayonet, which meant that riflemen could be, and were, run down and bayonetted by British troops after they had fired their one shot. Bayonets are not often used in combat today, but are useful as a psychological weapon in dealing with civil disturbances.
A fourth is lack of protection against a hot barrel. Most sporting rifles have no handguards, not a problem when firing one or two shots at a deer, but a consideration when a rifle is fired hot in battle.
Of course, one can take a sporting rifle and use it "as is" in special cases, such as sniping. Or one can make a sporter rifle easy to maintain, and put on a military stock, handguard, and bayonet lug. But then it is not a sporter rifle, it is a military rifle.
Today, there is considerable divergence. The civilianized military rifles are still basically military rifles, even without bayonet lugs. The true sporters are not intended for military use and are not likely to be used as such except in those special cases. IMHO, a Ruger Mini-14 or Remington 7400 would not stand up very long in combat even if provided with selective fire. The various bolt action and lever action sporting rifles would be hopelessly outclassed and unsuitable in general.
Jim