The answer is quite simple. The glock does not meet the US procurement specifications. I have not looked at the specs in some time but my memory is that the specs requires an external safety.
At the time the Beretta was adopted, the CZ75 was the favorite/best pistol in the competition. Unfortunately, it was manufactured in a Communist Block Country. Therefore, it was eliminated from consideration even though it preformed significantly better than the Beretta.
The 9mm vs 45ACP was decided on two issues. Politically the US wanted NATO to adopt the 5.56 therefore we traded the 45 for the 9. The second issue was logistics. You can pack a lot more 9mm in the space required for the 45ACP.
Historically the 7.62 replaced the 30-06 because it required 1/3 less space than did the 30-06, allowed a shorter action, and the existing manufacturing facilities required minimal modification.