There are many reasons why people choose a down filled jacket: it has a superior warmth-to-weight ratio, it is highly compressible, it has excellent heat retention and, with proper care, it can last a lifetime. Unfortunately, down has a weakness: moisture will cause it to lose its loft and all of its insulating properties, leaving you feeling cold and wet. Drying down takes a lot of time and effort and, when done incorrectly, it can damage the jacket.
Synthetic insulation is cheaper and will keep you warm even when it's wet. For this reason, particularly in damp climates such as the UK, many people find synthetic insulation more reliable and easier to look after. However, synthetic insulation has weaknesses too: It doesn’t retain heat as well as down, it tends to be heavier, which also makes it less packable, and it has a shorter lifespan than down.