I wouldn't doubt most if not all European country's law enforcement agencies purchase their weapons for their officers. Here in the US, many agencies still allow or make their officers purchase their own weapons.
True, but I think an equally important factor may be the fact that the USA simply
has more police agencies.
Although I'm not familiar with Northern Ireland, many European countries and other Western countries such as Canada have a powerful national police force and local police forces in cities, but that's basically it, and the latter is sometimes equipped by the former.
An area in the USA might have a local city police force, county police forces (often more than one- for instance, a sheriff's department AND a constabulary), a state police force, and then the federal FBI, which, unlike many European central agencies, is mostly an investigative service. Add numerous federal special investigative or protective services (Secret Service, ATF, TSA, etc.), the odd state-level special investigative service independent of the regular state police, and special private forces such as railroad police (who carry sidearms and have full arrest powers in many US states), and you're talking a LOT of different cops.
FWIW for various historical reasons, the USA was a relative latecomer to the idea of powerful centralized police authority; many of the state and federal-level forces were superimposed over local police in the early 20th century, but did not replace them, leading to a bewildering array of agencies.
Since these agencies are often run under different government umbrellas with different sources of funding, there's a better chance that some will be replacing sidearms at any given time, and that some will replace them more frequently than others. Keep in mind that gunmakers like to tout these programs in their press releases to boost their stock prices, and politicians like to tout these programs in
their press releases to show how they're being "tough on crime" and "protecting the citizens", so there's lots of publicity when it happens.