Why do fire extinguishers lose pressure over time?
No man-made container will ever be 100% leak proof over time. Even a Mylar balloon (the more expensive fancy ones) will lose it's helium over time and no longer stay afloat. Most fire extinguishers are charged with nitrogen under pressure as their propellant. Nitrogen is an extremely small molecule and given enough time will slowly seep or leek out of the fire extinguisher.

Changes in temperature and barometric pressure would be the culprits making this loss faster over time. Keeping them out of direct sunlight and away from heat sources may help prolong the amount of time until they need to be recharged. However, keep in mind there may be laws or regulations governing where a fire extinguisher should be located.

This is one Example

My father and uncle worked for years installing fire and security systems. Along with a schedule for systematically checking smoke detectors, you should include checking the charge on your fire extinguishers.