I think it was just too expensive to make. They made good use of unused clips from movies and old news reels, but you couldn't imagine anyone doing that today, and period sets and costumes are costly. Doctor Who suffered the same, which I expect is why there were so may episodes of that running down spaceship corridors and fewer on location in slate quarries. I agree with you about the potential. I really liked that series as a kid. I think that the not bringing them back home was a way to allow the episodes to be show out of order. That was very important with syndicated shows in the 1960's that might be on several different US stations, but I would certainly have liked to see a story arc. If they had been able to have developed further the Russian "secret weapon" and had had a time tunnel "space race" between the USA and USSR.