Hetzner has been an established player in Europe for a long time. It seems plausible that they have enough customers who use small amounts of bandwidth to subsidise the heavier users.
Considering switching costs, if they enter the US market with better pricing than established players, it stands to reason that the customers that would be most enticed to move will be the heavier users.
Europe is also a lot smaller network wise.
Hetzner only have to get their traffic to Frankfurt to get connected to practically the whole of Europe. For the US, Ashburn N.Virginia is good but it's still only a single coast.
