After the cancellation of 'Star Trek', American TV suffered a drought of SciFi. Only a few, low-budget, short-run, American shows were on TV.
The Brits filled that gap with great shows like 'Dr Who', 'UFO', 'Space 1999', and other not-so-great shows like 'Star Maidens'. Most of which could only be seen on PBS.
It wasn't until the unexpected popularity of 'Star Wars' in 1977 that American sci-fi came seriously back in the air.