They don't issue Glocks as a general duty gun because they have had a contract with Beretta since 1985. Back in the early 80's the military set forth specifications for features they wanted in a new handgun. After testing various guns the Beretta and Sig passed all of the requirements and Beretta under bid them. Glock was not in the running because they do not make a gun that met the specifications.
You can bet Glocks are used if needed/wanted in Special Forces applications. As well as most any gun. This is why some 1911's, S&W revolvers, H&K's, Sigs,and yes even Glocks are often seen in soldiers holsters.
If/when the military decides it wants a new handgun, or any other piece of equipment. they will write up the specifications they are looking for. To be honest they often pre-determine which product they want and write the specs so that only the option they want meets the specs. Any company that can make a gun that meets all the specs and will sell at the lowest price wins the contract. A Glock could be the next military contract if the specs are written in different way, but not with current requirements.