I think for me, one big reason is just how mechanical it is. Ever since I was little I loved watching complex machinery run. I could spend all day in games like Myst and Riven just watching what every different switch and button would do. Pinball was something I didn't have a ton of interest in until I first played the Williams Collection on Wii. Seeing Rudy and knowing he actually existed in real pinball machines I think really intrigued me. After that, I learned that these games had more to do than just randomly flipping at targets, thanks to the in-depth instructions in the Williams Collection. I can't remember for sure, but I bet it was moments like my first jackpot on Funhouse, hearing Rudy scream and the music change, that got me hooked.
So for me, it's watching how the machine interacts with the ball, trying to complete the challenges the game sets before me, and the excitement I feel when a machine starts practically exploding with lights and sound (I'm looking at you, Twilight Zone). The final aspect that *keeps* me hooked must be the unpredictable nature of the ball. Arcade video games don't keep me playing very long, and I really think it comes down to how littler variation they have. If I play multiple games of Donkey Kong 3, they'll mostly all feel the same, except that I'll probably gradually get a little better each time. With pinball I can have one good game, then a bunch of horrible games, and then I'll end the night with a new high score, all the while still feeling responsible for my performance.
So that's why I love playing pinball. Don't get me started on why I loving owning a pinball machine!