I know I'm late to the discussion, and much of the reasons were already given, but I'll succinctly re-iterate several of them:
1. Short lived show (11 months)
2. Until recently, was never aired on TV since it went off the air 41 years ago.
3. Randy Amasia (he single-handedly put the show in the Internet map)
4. Bert Sugarman's long-term reluctance to re-air the show (word was that he wanted a king's ransom to have them aired again)
5. Limited number of episodes available for viewing on YouTube (and of those, most IMO really didn't do the show justice as to how good the gameplay was).
Then, when you factor in that the actual game takes roughly 4 minutes to play (add 2-3 if you want to include the blockers soliloquizing their traps), with humor, knowledge, and strategy being executed within 60-second blocks of time, Whew! was a highly original game with a very unusual format - even if the rules were a bit complicated and it would take a moment to catch on to how the game was played. The cartoon opening, colorful set, Scooby-Doo-ish looking villains with their one-line quips, and fast paced music (thank you Alan Thicke) helped give the show its charm.
I saw the show when it originally aired in 1979-80 (I was 5 years old at at the time; it was appointment TV for me, especially during the summer of '79), and it is definitely appointment TV for me now; as a matter of fact, it is probably one of the very few game shows I've seen in my life that I tune in to watch daily.
In short, Whew's cult status is attributed to the fact that the show is original - and it is actually a very good show; it's just that the declining ratings and CBS really didn't do the show justice, especially when they converted to the celebrity format.