It's all politics. Its a big financial commitment for whichever side goes first. The US has a bigger population than Britain. And perhaps the industry politics were different there too. The card companies may have got their way.
There's liability to consider as well. In the US, the consumer is generally not liable for fraud; I've certainly heard that in other countries with chip and PIN, if somebody steals and uses your card (having somehow obtained your PIN) you don't necessarily get your money back.
