First, it was assumed that the AR rifle (aka M16, M16A1, M16A2, M16A4, M4, M4A1, etc.) is primarily what the military was thinking about when they adopted the M17 (Sig Sauer P320). They are both “modular” weapons that allow tremendous flexibility. I believe the modular criteria for evaluation was written into the competition specifically because of the US military’s experience with the AR. Modularity is one of the main reasons that the AR has been the US military’s primary rifle for the past 52 years. It has allowed all sorts of changes and upgrades without having to adopt a new weapon. I believe they expect to get the same flexibility and long term use from the Sig Sauer, which was the only pistol in the competition that was modular.
Second, ironically the Beretta M9 was adopted by the US military in the same year that the Glock pistol was introduced into the US market—1985. The Glock embodied two new technologies that have come to dominate the US handgun market over the past 32 years: (1) a polymer body with steel parts used only where needed, e.g. the barrel, and (2) a striker fired action, which means you only have one type of trigger pull rather than two—first a double action pull, followed by a single action pull such as in the old school Beretta M9. Both of this technologies are considered real advances by many, and the new M17 has both of them.
There are some reasons why the Sig Sauer P320 outperformed the Glock in the bid to be the US Army service pistol. Firstly, it was shown to have a “slight technical advantage” over the Glock – the Sig ultimately scored a “good” rating over the Glock’s mere “acceptable” in the U.S. Government Accountability Office’s report.
In addition to that, the Sig won a lot of points in terms of ergonomics. Sig Sauer makes use of things like textured polymer grips to counteract moving under recoil, and this goes a long way. It has also to be noted that Sig did undercut Glock’s bid by a wide margin – $103 million less to be precise. They also offered a two-gun proposal the XM17 and compact XM18 that outweighed Glock’s single-gun bid.
While it is sure it was more practical in terms of financing, Sig Sauer generally provides excellent firearms that perform much better than others in the same category – take the P365 as an example. Because it’s much lighter, it also makes a lot more sense to carry a Sig all day every day than a handgun that could get too heavy with prolonged carry.