In a 1989 article for the Fanzine "625" Tim Munro spoke of his disgust at the BBC's Head of Drama (the man responsible for making overseas purchasing decisions) openly stating at a press event that he thought that Star Trek was "TERRIBLE" and that it "might pop up on BBC2 at some point in the future". He went on to admit that although he'd only seen "a few minutes of the pilot" that that was more than sufficient for him to have made his decision.
This is, of course the same man who (with the open support of the notably sci-fi phobic BBC Head, Michael Grade) was nearly successful in permanently canceling Doctor Who.
It's worth noting that a considerable number of US shows never make it to the UK. Those that do are generally those that have managed to build a strong following in America and have a proven affinity to a key demographic. In the case of Star Trek TNG, it was launched in America and then gained a substantial impact among 18-35 year old males, making it a very worthwhile purchase for overseas buyers such as the BBC.
Buying an un-tested show, with the inherent risk that it flops on both sides of the Atlantic is something that cautious buyers have generally wanted to avoid.