Recently, in the "will Looney Tunes ever find success again?" thread, DrTooth mentioned that it seems like the classic Looney Tunes shorts were always more widely shown on television, but the classic Disney cartoons were just a little more popular. And in time for Halloween (okay, a little late, but I did think about this a few days before Halloween, just didn't get around to bringing this up here until now), I feel you could say the same about The Addams Family and The Munsters.
I feel like The Munsters was always a little more widely rerun while The Addams Family was always a little more popular. And it could just be me. It took me a while before I was able to see full episodes of The Addams Family, while Nick at Nite began airing The Munsters half a year before TV land (which reran the show when the channel was launched) went on the air (and my cable provider didn't pick up TV Land until a year and a half later). And even after I got TV Land, I feel like I still watched The Munsters a lot more than The Addams Family.
Of course, The Addams Family not only has the luxury of being a hit television series (though both shows only lasted two seasons), but also had two successful theatrical feature films (in addition to a made-for-TV movie and a made-for-video movie), while The Munsters movies don't seem to be that popular (with the only theatrical one being released around the time of the shows original run). Also, The Addams Family had two animated series, while The Munsters never had an animated show. There were also a number of Addams Family video games (though I haven't been too familiar with them, I mainly know about them because of The Angry Video Game Nerd's review of Fester's Quest, though now I'm remembering that the original video release of one of them included a promo for an Addams Family video game). And of course both shows had television revivals which aren't as well-remembered, and both shows had iconic theme songs. And the fact that The Addams Family started as a comic strip could have helped its popularity (though I think it's origins as a comic are a little obscure).