The US is much larger than individual European countries and has an order of magnitude more credit card readers, etc. in circulation. Every single one of those had to be updated to use the new system.
I don't know why you would need to activate them all on the same day (and in fact that did neither happen in the US nor anywhere in Europe). An incremental rollout is always possible and the size of a market does not change anything about that. My European credit card still works with legacy systems deployed somewhere in the world that hadn't been updated. That doesn't change the benefits they got from updating most system here.
Incidentally, the large schemes usually set Europe-wide rules anyway.
Partly that's because the US banks didn't think the EU standard was secure enough. They were proven somewhat right when a fraud ring found a way to bypass some of the authentication[1]. Long story made short: The EU system did something like authorizing the pin to the card using the chip, but not requiring the bank to authenticate, which allowed the terminals to function without a network connection. Criminals inserted the chip from one card over the chip from another, intercepting the pin authentication request to the card and allowing it, and then allowing the transaction to proceed against the regular card. This would not work in the US where all the requests are verified by the bank over the network (which is why it takes longer).
That the US card industry is trying to move away from requiring a pin is somewhat strange, but I think can be explained by different people weighing in at different stages. Engineers nixed the EU type system as not secure enough, and then different company decision makers start pushing for no pin, to make sure they don't see a dip in usage. What you end up with is this.
But you don't have one or the other, you have both. Then you can upgrade slowly. It's not a great example (very small) but that's how it was done in New Zealand.
