It was 100% the streamline and approachable mechanics. While Oblivion was popular to a point, it still had some hurdles to bring in the casual crowd. Leveling up and character building still had relics from past entries such as Major and Minor skills + Class systems and attributes. You had to be careful when leveling up, you could easily end up with very low attributes and high skills, which made the game a bit daunting to play for too long. Skyrim made combat much more enjoyable from Oblivion with more fluid actions, add in the fact your "class" is whatever you want it to be, and be able to switch up your play style any time you want. You had tons of freedom to do almost anything you want in the world with no real repercussion. Approachable controls for console players, with menus that weren't cumbersome (Unless on PC. Thank Talos for SkyUI). Of course add in Dragons and other surface level things like shouts, Nordic themes and a cool new environment. Follow all that cool launch trailer that got everyone excited, and you had a game that would invite nearly any player in, and give them a warm welcome and be able to keep them invested.
Skyrim wasn't my taste when it launched, but even still, I'll admit sinking hundreds of hours into it. Coming from Morrowind and Daggerfall, the in depth RPG mechanics are what I wanted, and the elaborate and insane amounts of choices for role playing, Skyrim had significantly less than those games. However, what I got was something else that still kept me wanting to play more and see more.
Even now I'm currently replaying it to see how my opinion has shifted over the years since it's initial launch, and I'm still finding myself enjoying the game for the same reasons I did back then.