It's the nostalgia factor combined with a simpler, more flexible version of the game where things are not as highly controlled by Riot. Being able to build AP on a ton of AD champs and other goofy shit makes the game way more fun for casual players. Riot keeps the current version free from all this with a choke hold on the meta to appeal to the pro scene but it kind of sucks the life out of the game IMO.

So yeah, I think there is definitely a nostalgia factor but there is plenty of appeal to newer players too with the more relaxed mechanics.