Since it was my suggestion I'm compelled to answer

Half of what you mentioned is "eastern half" already, and the other half didn't have timber in abundance, not nearly enough for their needs. With the exception of Germany, most of Western Europe became quite deforested during the middle ages, especially Britain (we're talking about quality timber such as oaks not just any forests). There are historical accounts of early shortages of timber for shipbuilding and construction (see
this for instance). It was therefore imported from Scandinavia, Baltic area and Eastern Europe, becoming one of the major commodities traded by the Hanseatic League.
This change facilitates in-game trade of a resource that was previously abundant everywhere and hardly "strategic", and it reflects the Western maritime powers' struggle for shipbuilding timber and historical exploitation of Baltic/Eastern European forests...