I agree with Buffnut. Additionally Id say cost and labour. Using wood in ship design requires a skill set not required to work aluminium. Relatively easy to build a frame in aluminium....harder to fashion a skin that was waterproof. You also need rolled sheet to fashion the skin, which may well have been a production bottleneck or required added factory space and cost that the germans did not want to pay.
as a rough gneralization, it is far far chaeaper to build ships huls out of wood than it is out of steel, including aluminium.