Designed partially and built mostly by Germany, hence the German tree.
We have made it clear several times. Nations without a tree have their vehicles distributed based on who they are closest too or what gap they fill.
For example Class 3P is developed and built by Germany mostly, so it goes in the German tree. Rooikats were more British, so they go British.
Canadian ADATSs would normally be on the fence and not really something we would generally add because of that, but British tankers wanted a new SPAAG and this was one of the top viable suggestions, plus America already has an ADATS system, so the UK was even more logical.