From what I can tell, some of the greatest shows that were canceled early on and then came back to cult greatness were only around three seasons. I mean, Star-Trek, Gilligans Island, Family Guy, Arrested Development (which only just got cancelled but I know its gonna get big again!). I know there are more out there. Just can’t think of them off the top of my head. So, what do you think? Is it something about a three year series? What are the other great shows that I’m forgetting?
I think it’s a bit of a selective effect. Three seaons is about the length of time a show would last that had enough of a fan base to be a cult hit, but not enough to keep being produced.
Gilligan’s Island only ran for three years? Seemed like forever in reruns…
I think it may also be that you have to suck really bad to suck before three years. That is, there are no bad Arrested Development episodes. None. They didn’t get time to start sucking. Give 'em another season or two, and there might have been some duds which would have diluted their cult.
But I think you’re misunderestimating some other “cult hits”, like Firefly, Sports Night, My-So-Called-Life and Freaks and Geeks, that never even made it to three seasons. I could just as well ask about great shows only lasting one or two seasons but then gaining a cult following.
Last longer than three years and have great DVD sales, and you’re not a “cult hit”, just a popular show: Seinfeld, Friends, etc. Studios are rarely willing to pay for more than three seasons of a show that hasn’t found a large audience. Buffy and Angel are the only two shows I can think of that lasted more than four years but are still referred to as “cult faves”. I’m not sure why that is.
How about syndication helping make a show popular? TV stations have a lot of time to fill and they can repeat episodes over and over. Independent stations used to thrive on all of the 1960s repeats that filled the airwaves, especially in the afternoon and early evening.
Dark Angel only got two seasons! 
And the second season was the first season’s evil clone…