Anthologies and series are really two different kinds of shows. As a series, committed to a repeating cast and setting, Star Trek was the best series. Twlight Zone was the best non-realist anthology. But it had plenty of dogs and tough competition from The Outer Limits.
This is exactly how I'd view it. And while I'm not a writer, I would imagine that it's easier to have a regular rota of continually turn out independent, self-contained and unrelated stories (brilliant one, it must be said) for an anthology show than to get writers to write stories set within the confined settings of a regular series with its own premise, setting and 'rules.'
Plus for all the undoubted brilliance of TTZ, I don't imagine that many fans can have the same sort of affection for it, the way that they do for Trek, with its regular characters, its iconic spaceship and optimistic take on humanity (though you can't deny the uniqueness of Serling's own vision).