I love a good 1911, but they are not the end all be all...
They have limitations... Mostly in the lack of capacity, higher weight, and higher maintenance requirements... I have read more than once that the M9 was more reliable than the 1911 as well. Even if we give the 1911 some leeway and say it is as reliable as the M9 was, it still has its other deficiencies.
We can make a reasonable assumption that the reliability and performance of the Sig 320 was similar to the M9... I doubt it was vastly superior, it was the features, ergonomics, and lighter weight that mattered... And resulted in the selection.
As far as the points above about the 1911...
More capacity is always better than less. (Provided you stay within the bounds of reason, of size and weight)
Lighter weight is better than heavy... Because you have to carry it around a lot.
Easier maintenance at the individual and armorer level is better...
45acp really isn't much better if any, at stopping a threat, than 9mm... HP or FMJ... Hitting a vital area is the most important thing, a slightly bigger hole isn't going to make a drastic affect, if any, on bleeding rate... all else being equal. The more bullets, the more chance to hit a vital spot. Also, more bullets means a better chance of defending against more enemies.