Thanks all for your great responses. My key takeaway from all your responses are:
My interest is logical and not historical - most of the literature seems to imply that the fifth postulate is more questionable in an absolute and obvious sense, which is what confused me. This discussion has clarified that it is not obvious.
1. There are several nonscientific (historical - Euclid didnt introduce it for a while, aesthetic - it "looks" uglier) reasons why it turned out that the fifth postulate was debated more than the others. 2. The negation of the fifth postulate led to practically useful geometries and thus came to the forefront. 3. There is active work in logic that investigates negating the other postulates as well, based on (Bolyai et al's) further abstraction of denying the common meanings of line and the plane.