"I just really don't understand why chip & PIN is such a problem in the US."
Because the US has A LOT of rural areas where you would be amazed to find they still use imprinters...because even access to a telephone is not guaranteed (heck, they may not even have electricity). Chip readers would be useless to them.
"...so why are their retail and banking industries so reluctant to use a technology that has been demonstrated to reduce fraud in Europe and elsewhere?"
Because fraud isn't really that big an issue in the US. Well before Chips, the US has had a robust chargeback system in place, and the credit card companies have ways to investigate frauds. Given the volume of activity in the US, the overall percentage of fraud is close to the noise level, and most of that is not of the type that Chips can help control (for example, e-tailer or CNP fraud).