This wasn't just a fight where Holm got in a lucky punch and won by punchers chance. It wasn't even a fight where Holm was just decently better than Rousey. Rousey was pretty much considered the greatest female combat sport athlete in history by a wide margin and Holm delivered what only can be considered humiliation to her.
There area a couple of things that stand out:
1. How thoroughly terrible Rousey's game plan was. She must have thought she was always going to walk over every opponent she was ever going to face, easily out muscle them, or be able to get them to the ground at will. She had no plan B, and her plan A sucked balls. Fighting to an opponent's strength and just lunging your face into their fist when they can strike and move a lot better than you is plain stupid. Being able to clench or shoot in sound all well and good, but when your opponent has better speed, timing, and spacing than you and punches you in the face hard each time you go in it becomes not so easy anymore, especially when a fighter can't use their own stuff to set things up. It's just one of the many reason a fighter should work to be great at all aspects of the game, because they all work off of each other. Nobody that's smart in a sport ever thinks of themselves as invincible, especially in a combat sport where unpredictable things happen all the time. Which leads me to the next point.
2. I knew striking was Rousey's weak point, but geez, bad doesn't even begin to describe it. She was finally forced to use it by somebody that could defend against Rousey's strengths well,...and to be blunt her striking is down right amateur. It's not just subpar. It's embarrassing. There is no way a full on professional fighter of that caliber should be that bad, especially with the money and resources Rousey has and as long as she has been in the sport. Off the top of most people's head they would have to go pretty far back in UFC history to find champion caliber people that were also that bad at it. She got away with it for a long time because she was just that much more physically dominate over other people she faced until this match, and she, while being a one trick pony, at least had a very awesome trick. Then she finally wasn't able to use it.
3. Maybe it was the beating she took, maybe it was the fact she almost never had to fight past the first round, or maybe she didn't take this fight as seriously as she should have, but Rousey was gassed by the end of the first round. Her conditioning wasn't as well as Holm's. You could tell just by looking at them.
All that said it's nothing to take away from Holly who just beat the crap out of her from start to finished and deserved the title, and was much smarter than Rousey this fight. As dislikeable as Rousey has gotten lately when she isn't in front of a camera for a mainstream television show or movie I have a hard time feeling sympathy for her.