I know the coal fields of England are also from this period. I gather that almost all the commercially exploitable coal fields in the world were laid down during the Carboniferous Period (which in the US is usually divided into the Pennsylvanian and the earlier Mississippian).

A lot of factors go into it. The low sea levels caused by glaciation also exposed vast swaths of former seabed to become low marshes. Also the plants evolved lignin (the stuff that makes wood wood-like) which is hard to break down, so a lot more plant matter persisted to be buried.