Question

why is frontal rainfall common in subpolar low pressure belts

Answer 1

it's not common rain it's acidic rain

Answer 2

The Subpolar lower pressure belt is created by the confluence of cold wind masses from greater latitudes and heated air masses from moderate latitudes. In the north region, their confluence forms the polar front, which allows the low-pressure cyclonic disturbances to cause the precipitation in the Pacific Northwest and many parts of Europe hence rainfall is common in these areas.