Analog signals have an uneven power spread in the signal, causing a spike, which is detectable even from long distances. It's how we use our own giant radio receivers to listen on what's happening in deep space. One you find a spike you can adjust your receiver until you get the perfect signal. Think of it like back w/ analog signals and rabbit ears even if you weren't right on the frequency you could still see what was going on, sure the picture wasn't super clear, but you know, it was fine. Of course if they don't know what they're looking at, the aliens might have a problem with a not-so-clear picture and wouldn't really know if they were making the image cleaner or worse.

Digital signals are evenly spread across the spectrum, so no spike means its harder to find the signal from deep space. You can sort of see this when you begin to lose a digital signal, it starts to freeze/break up like a scratched up DVD does .. there's no way you can "sort of" get the signal, you either do or you don't, on or off, 0 or 1.

So it's still possible to catch the signal but it would require a huge antenna as pointed out by Danny, one the size of Texas is an example (though not sure if that's actually true or just in jest), because now it's like finding a needle in a haystack.

All of that wasn't the best of analogies, but I just wanted to point out that analog signals are "dirtier" and so easier to find in the whole mess of radio waves in space, whereas digital signals can get lost in that same mess of radio waves since they're so clean.

Finally I'd like to point out that our alien friends are probably many years behind us on television (limits of light speed travel applies to radio waves) so we should probably make sure they don't find out about the Oceanic 6 because they'd be pretty pissed.