Because neither of them lasted at all which means they sucked.
UFO didn't last because it was too expensive to produce with all the effects for the limited UK market.
In fact, Space:1999 was the US attempt to give Gerry Anderson enough money to make a scifi show as good as UFO.
In this he failed.
btw Gerry Anderson also produced Thunderbirds
From wiki:
Two years after the 26 episodes were completed, the series was syndicated on American television and the ratings were initially promising enough to prompt ITC to commission a second season of UFO. As the Moon-based episodes appeared to have proven more popular than the Earth-based stories, ITC insisted that in the new season, the action would take place entirely on the Moon. Gerry Anderson proposed a format in which SHADO Moonbase had been greatly enlarged to become the organisation's main headquarters, and pre-production on UFO 2 began with extensive research and design for the new Moonbase. These developments were not without precedent in the earlier episodes: a subplot of "Kill Straker!" sees Straker negotiating with SHADO's financial supporters for funding to build more moonbases within 10 years. However, when ratings for the syndicated broadcasts in America dropped towards the end of the run, ITC got cold feet and cancelled the second season plans. Unwilling to let the UFO 2 pre-production work go to waste, Anderson instead offered ITC a new series idea, unrelated to UFO, in which the Moon would be blown out of Earth orbit taking the Moonbase survivors with it. This proposal developed into Space: 1999.
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