1. The books are older than our parents and everyone has heard of them in some way, shape, or form.
2. It was well marketed and not overmarketed (read: The Hulk)
3. The production process has been well documented and without the typical sorts of problematic reporting that came with say Titanic or the Matrix sequels. People knew what was going on and they never really heard anything negative about the whole process.
4. The CGI do a great job in the story. I agree they do not tell the whole story like some people claim they do or like the SW PT movies have done, but they are a very important element in the popularity. I think the popularity of Gollum is what's continuing to bring people in for ROTK. Many may have missed him the first time around and now, after seeing his "acceptance" at the MTV Video Awards, they were completely entranced and want to see him on the big screen finally. MTV may suck as a whole, but you cannot deny it's power over Young America and the rest of the world.
5. Sex appeal... Liv Tyler and Miranda Otto for the guys (and not-so-straight women) and Viggo Mortenson and Orlando Bloom for the women (and not-so-straight guys). A lot can be said for having good looking people in a movie. Even if they are not that well known like Otto, Mortenson, and Bloom were before the release. Mortenson is known as an actor but he was never a "sex symbol" per se. If anything, he was a rather greasy looking kind of guy. But, Jackson redid a lot of the story to allow for a female-appeal love triangle with Arwen, Eowyn, and Aragorn and it helped immensely to bring in women.
6. A multinational cast of epic proportions. Brought in people of all cultures because they want to see some of their fellow kinsmen. Unlike the Harry Potter films whose only non-UKer was Verne Troyer (Mini ME in a cameo), these have the appeal of having people from around the world cast in it.