They have different commercial break structures for one, along with an entirely different structure to their unions and they're royalties overall, so unless it's packed into an ad-friendly format like Benny Hill and Monty Python were, it's usually passed on. There's also 'oh no scary accents!' syndrome that makes them a hard sell (though all those Discovery reality shows with bad mic work needing to be subtitled/lots of mumbling are much worse).
But the other thing is a lot of folks have long figured out VPNs to watch direct from overseas without the obnoxious sell-throughs many syndicated shows have now (theme weeks and 'special offers'), and UK-specific streaming services now exist in the US, so that's where their focus went long ago, along with public television. It's also why BBC America is just an American rerun farm now, because AMC's budget cuts have killed their acquisition budget.