A couple of commenters asked for my "take" on why sociology is reputed to be a relatively easy major at many colleges and universities. Okay, why not? I'm just to going to write it as a series of observations and conjectures, though:
1. The intrinsic substantive difficulty of a field, whatever that means, is not a good place to look for the explanation of why some majors are easier than others, as instructors have the capacity to vary greatly the difficulty of courses on the same topic. There was a minor scandal many years ago at the University of Iowa over all the athletes who were talking "Watercolor" because it was an easy course to get a good grade in. The craft of watercolor itself is (I hear) very difficult and a course on it presumably could be made extremely hard.
2. That said, so many people are so resolutely incorrigible and freaked out about math, it's probably safe to assert a tendency in which the more a substantive enterprise involves math, the more difficult a major in that enterprise will be perceived as being. Most sociology dissertations are not quantitative, so it's not surprising there tends to be minimal math in sociology classes.
3. With varying directness at different universities, majors are a source of revenue for departments, so one might expect the "equilibrium difficulty" of a major to be whatever maximizes the number of students. All else being equal, it might be simpler to explain why an easy major would attract more students than a hard one, so perhaps difficult majors are bigger explanatory puzzles than easy ones.
4. One reason a student might choose a hard major over an easy one is that the hard major is perceived as offering greater career rewards. Any major that has to put up bulletin boards for undergraduates about "What can you do with an X major?" is more likely to be an easy major.
5. I'll conjecture that departments less dependent on students for revenue because of grants and other revenue streams tend to offer harder majors. Sociology has only modest funding from grants. BTW, if true, an implication might be variation in difficulty of majors is higher at research universities with considerable grant revenue (spread wildly disproportionately around departments), than at liberal arts colleges.
6. "Sociology" is sufficiently mysterious to high school students that very few enter college with the idea of majoring in sociology. Sociology commonly picks up many majors who only decide to major in sociology relatively late, and after taking some course after intro. For this reason, sociology has a greater cost to trying to impose any kind of prerequisites on students. Multicourse sequences are more difficult than courses with no prerequisites, especially as the latter often have to offer redundant conceptual content with one another.
7. Professors differ in the psychic rewards they derive from being seen as tough, presiding over a tough course, telling classes on the first day that a third of them will either drop or fail, etc.. Sociologists tend to be less stoked to give large numbers of students bad grades than practitioners of some other disciplines I could name.
8. Sociologists are, professionally, better at generating non-individualistic explanations for individual failures. So they tend to be constitutionally more squeamish about making distinctions among students, which inflates grades and makes the major easier.
9. Consistent with a broader aversion to hierarchy, sociology draws disproportionately from the pool of those who would be fine abolishing grades altogether. I'll conjecture the prevalence of this attitude in a discipline is correlated with how easy it tends to be. Any discipline in which a leading textbook is titled "Down-To-Earth X," or something equally hippie-friendly, is more likely to be an easy major.
10. Even after all the foregoing, I would want more evidence before I conceded that sociology is typically an easier major than most majors in the humanities or fine arts. Sociology is identified as a "social science," though, so when people call it an easy major, the humanities are not the comparison group in mind.