My impression was that because of the PIN used it was a lot riskier for the consumer with European cards: the credit card companies essentially said "the PIN was correct, so it could not have been fraud" so the default assumption was no fraud.
In the U.S. the system favors the consumer, so the merchant has to actively prove that it was not fraud (by default, this is definitely not blindly applied).
The signature part is one part of this, but the U.S. actually has laws in place to push this, and that largely has driven the non-PIN policy in the U.S. (since most of the anti-fraud pressure is on other parts of the system).
In the U.S. the system favors the consumer, so the merchant has to actively prove that it was not fraud (by default, this is definitely not blindly applied).
The signature part is one part of this, but the U.S. actually has laws in place to push this, and that largely has driven the non-PIN policy in the U.S. (since most of the anti-fraud pressure is on other parts of the system).