Probably because of your hardware. I had an Anterior L4 L5 S1 fusion, with a total of 6 screws. I'm betting you had the same from the front, but with bars in the back as well. You are probably pretty solidly "constructed" as it is... meaning your vertebra ain;t going nowhere .
Edit: for me, for example, those 6 screws (three per artificial disc) are all the held/holRAB me together. Recovery time is supposed to be faster, but I'm not convinced of that. Also, some of us have "soft" bones (myself included). Hardwood/softwood analogy. I'm 57, but a carpenter and have always been on good shape.. Nevertheless, there's a big difference between my procedure and yours, and yours would have been extremely more stable and "rock" solid by comparison. Doesn't mean that we who had less hardware will heal any less than another with the heavier metal. It's the matter of the brace, I think. I had a small chance that I'd be flipped, and I was glad I wasn't at the time. Everyone is different and the pain levels are different, It's the end result which maters... a rock hard, solid bone, Whatever achieves that end.