I see this claim a lot and often wonder if it is true. It certainly is true that the original series was a great success in syndication, but did it really have more viewers in re-runs than first-run? I don't doubt that, over time, more people saw the series after it was cancelled than had watched it on NBC, but was there really an audience at any one time that was bigger than the one delivered by a major network during the three network era?
I'm not sure it's even possible to make such a comparison, since in its network run it was run in a single weekly time slot, while in syndication it aired on different days and times in different cities. So you'd never have as many people watching any given showing, because that would only give you one city's worth of viewers, maybe a few if their schedules converged. I suppose you could compare the total number of viewers it had per week, but I'm not sure that would be a meaningful comparison given the different circumstances.
Anyway, I think it was more a cumulative thing, seeing the episodes over and over. Because the number of episodes was smaller than a typical syndication package, they got rerun more often, and that helped build more familiarity for the episodes and gave viewers more chances to discover them and get hooked. So the repetition itself was part of what created the loyal following.