Because in Europe they collect VAT on all new cars, which is basically how the corporations pay taxes, but when they export they get a full VAT credit. This effectively making all exported vehicles tax free to the corporation. Europe basically has a consumption based tax system, where as in the US we tax based on income. Because of the US tax structure we are actually at a disadvantage exporting to foreign countries.
They have also been absorbing the cost of the dropping dollar, but apparently have reached a point where they can no longer do it, so expect to see a price increase on European cars in the very near future.