The are going to the 40 because it is much easier to train someone to shoot the 9 especially if they have not shot a handgun before. At the range I work at many people going to the police academy show up to learn how to shoot before they go.
The only advantage the 40 or 45 has over the 9 is that it goes through window glass better.

Advancements in ammo have made the pointless compromise that was .40 even more pointless. It's snaps harder and takes up more space than 9mm, and it lacks the stopping power of the 10mm round from which it was derived.

The FBI is going back to 9mm as ammo is better now so police departments are doing the same

I've read extensively that 40 cal has a very sharp recoil that a lot of people really do not like; especially for target acquisition and such. That was supposed to be the new and improved 9mm but it seems its still the favorite

I just read an article by a senior ballistician, comparing .45, .40, 9mm. In short they are all very similar, but the 9 actually edges the other two out slightly when it comes to terminal ballistics. Smaller but faster, the 9 gets better penetration. Especially when the perp is wearing heavy clothes like jeans or leather. As for why officers are opting for 9 over .40, slightly more manageable? More round per magazine?