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First, there is a technology triangle involving treadwear, traction, and rolling resistance. To get better properties in one area, one or both of the others have to be sacrificed. That's why OE (Original Equipment) tires - the tires that come on new cars - have such a poor reputation. They were designed for good rolling resistance, and the treadwear and/or traction were sacrificed to get it.
Further "LRR" is a relative term. It means better rolling resistance compared to other tires with similar wear and traction properties. It does NOT mean absolutely low rolling resistance. In fact, many LRR labeled tires will have fairly high RR values - some higher than non-designated tires.
So why hasn't the US government required labels for RR (fuel economy)? Because the tire size plays a large role in what the value is and the regulators (NHTSA) want to use RRF (Rolling Resistance Force) which makes smaller tires look better (which also means overloaded tires), where the tire industry wants to use RRC (Coefficient of Rolling Resistance), which makes larger tires look better. The courts have ruled that NHTSA's proposal is unacceptable, but NHTSA has not yet issued a revise regulation.
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