It's all about cost!
Flint Michigan PD carries M&P 45's. Genesee County Michigan, in which Flint is in, carries Sigs. It's a tossup anymore between Glock, Sig and M&P's.
The US military is going to the Sig.
The 40 is disappearing fast, everyone in going back the 9 m/m. They can't get officers to qualify with the 40, as felt recoil is worse than a 45. Which is really nothing more than a lack of training!
The bottom line is cost! Who had the lowest bid a purchase time? As far as caliber it's also cost related, what is the cheapest caliber. And firearms training is the first thing PD's cut in a budget!
Plus, most new officers coming into the profession these days have no military background, did not grow up in a hunting or shooting family, so their first introduction to a gun is at the academy. Make it as easy on them as possible to qualify. If not the first time recycle them until they do.
It's hard recruiting police candidates these days, so make it easy for whatever you can get. What a shame, but then look at what they do to the good officers on the street these days!
Food for thought!