they also have an active GT racing competitions that attract a lot of media attention, sponsors and manufacturers (including the Falken backed R34 GTR). Setting up a branch in Europe will allow them to develop and refine their involvement in the series.
The US has a massive fanboy base, and like other Japanese companies like Apex, HKS and TRD will no doubt do well from expanding in there. Plus JGTC just set up a leg of their championship in the US, so it makes sense to be there.
We have V8 supercars... and people who love the bang for buck value of 15 year old cars. Not really much of a market for high end tuning house developed cars, and the profits to be made from just aftermarket parts is gonna be pretty thin.