Why do some police not carry firearms?

In some countries police are not authorised to carry sidearms at all times, and must keep them in a vehicle safe until required.

Added: This is particularly applicable to those nations which historically have a long history of obedience to the law, and a national psyche for doing so. Sadly, growing less-and-less common in this day and age.

NOTE: (in the US) ALL sworn law enforcement officers are routinely armed at all times.