After many years of owning lots of firearms of different flavors, then fine-tuning my interests to military pattern guns, the main factors in play are...
1) Commonality- Parts, mags, gear, support...when considering the ubiquity of 5.56/.223 in Military, Law Enforcement and Civilian arenas, one would be silly not to consider an AR15 from this standpoint alone. If the answer to this question was something else, then it would drastically change the rest of the considerations below.
2) Sustainability- Due to the aforementioned commonality, the long term sustainability of any gun should be a strong consideration. Here in the USA, if we assume the 5.56/.223 caliber as being the most common rifle caliber, then the AR15 follows as the most common rifle/carbine from which to launch it. You want to be able to have a chance at being able to easily replace any parts when the need arises.
3) Reliability- Assuming the previously mentioned AR15, it's there. Having been shooting since before these forums came about, I can tell you that there is overwhelming evidence that beat-up or homemade (Bubba) AR15s account for the vast majority of reliability complaints. Lots of other firearms are very reliable, but they fall short of the commonality and sustainability needs as previously mentioned.
4) Accuracy- In the case of the 5.56/.223 semi-autos, the AR has earned it reputation for accuracy. The real world results of shooters presenting real data are quite revealing. When a higher degree of accuracy is needed, there exists an array of components to meet/exceed that need. Not much more needs to be said in that regard.
5) Modularity- If you're gonna have one gun, it should be customizable to your liking. Aside from the typical THR haters, most everyone can make an AR into something that appeals to their needs or aesthetic sensibilities. Unless of course they simply hate anything with a pistol grip and removable mag. We've come to call them Fudds.
6) Weight- If you're gonna have one long gun for everything, then weight needs to be a consideration for any able-bodied shooter. For many years I've seen people lambast others for making weight considerations. More often than not, these are the people who only carry their gun from the car to the firing line. How many of them can even jog a mile? We're talking one rifle for everything, right? Things that make you go hmmmm.
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I've noticed that most people who would thump their chest and vehemently dismiss the AR haven't run a properly built gun, with proper lubrication, through its paces to even attempt to get an objective baseline. Let's just say there's a good reason why nothing else has displaced it for general issue.
