Why, you might well ask yourself, is there such interest? Well, put simply, Zep are the greatest rock band of all time; they exist in a class of one. Before you start screaming "What about the Beatles/Stones?" at your screen, I'd better get some definitions out of the way.
The Beatles weren't really a rock band at all. This isn't intended as any sort of criticism whatsoever, it's just a statement of fact. Their earliest material aside, The Beatles' music engages the mind and the emotions rather than the body. Or in other words, it doesn't make you want to pull a funk face and shake it. If we're getting right down to basics, it's brilliant but it isn't sexy. It doesn't rock.
The Stones present a bit more of a problem. Their best music rocks in a way that is undeniable. Yet they don't belong in the same musical category as Led Zeppelin because the Stones are ultimately a pop band. I absolutely don't mean this in the negative way that phrase is often used. Pop is short for popular, that's all, and the Stones are perhaps the greatest ever practitioners of assimilating diverse influences (everything from blues to country to disco to funk), putting their own stamp on them, and making them popular.
However, one thing the Stones have never done is define a genre. Zeppelin, on the other hand, have because Zeppelin aren't just a rock band, they ARE rock. They took the music that began with Chuck Berry, Little Richard, Jerry Lee Lewis, and of course Elvis (rock and roll, basically) to its logical conclusion.
They took its constituent parts of swagger, power, raunch, and volume, pushed every one of them to the limit, and in doing so left nowhere for anyone else to go but sexless parody, which is a nice way of saying heavy metal (Zeppelin were NOT metal - they may have helped spawn the genre but they didn't embody it).
This isn't to say that nothing noteworthy has happened since. Punk, New Wave, Indie, Britpop and all the other guitar-based forms which fall under the umbrella term of 'rock' have produced some wonderful music over the years but they invariably lack the quality that makes you want to pull your funk face and, frankly, get jiggy with it.
Keith Richards is fond of saying, "Everyone talks about rock these days; the problem is they forget about the roll", and he's absolutely right.
However hard they rocked, Zeppelin never forgot the roll.