You tell me. I mean, why are you asking other people why a certain game is scarier for YOU over the other? Art is subjective and horror is not an exception. Some things and how they are executed very well may scare you more than others, while it can be the opposite for someone else.

I, for one, or course didn't play DSR yet (can't, lol), but did play both Isolation and the original Dead Space. Both scared me, both delivered a tense, stressful experience, but Isolation for me personally was and still is more terrifying.