For starters, since its what I fired while in the service there is that apeal but more importantly are the following.
The aftermarket calibers make it very versital, such as the 204 ruger kit or the 50 caliber kit. (not the big 50 cal sniper but smaller case)
Accuracy in the new models with good ammo are impressive. My neighbor has one and he keeps 1/2" groups at 100 yards no problem. I know the ones I used while in the service were not that accurate for my likeing but most of us did shoot marksman so it wasnt all that bad.
Here in Illinois I have several opportunities for open prarie shots on running targets. That makes the AR15 (.223) with open sites a perfect candidate. Recover on target is quicker than the M-14 and the .308 is overkill for what I am looking to put down
, if there is such a thing.
Couple that with the significant cost difference (reloading), as well as availability pretty much anywhere the .223 wins.
Now, for the other application I am wanting to fill, its the M1-A1 that Springfield makes. Far more versital for hunting applications but the cost is outside my current budget.