I think the game objectives that fit to average players and more skilled players.
It provides enough targets form the average player: beat (not only play) multiballs, destroy the ring, win modes and collect elf gifts, get or beat taba.
For for more advanced player: valinor. I know a some people that have a lotr and the game does nor leave because they have not got to valinor. I think that is the most fascinating issue. Not to have achieved the final wizard mode. „One day I will get to valinor.“ Once you beat the final wizard mode of a pin frequently, it will get boring very soon.
I have not yet achieved valinor. Have been close (only MB ROTK and one elf gift left), but not yet there. I absolutely love the game (music, call outs, depth, objectives, strategy). But the main reason for the success of the game is the complexity to get to valinor.