"arXiv moderators expect submissions to be of scholarly archival interest to the communities they represent. A submission may be declined if the moderators determine it lacks originality, novelty, or significance.
Submissions that do not contain original or substantive research, including undergraduate research, course projects, and research proposals, news, or information about political causes (even those with potential special interest to the academic community) may be declined." as explained in https://arxiv.org/help/moderation
If your supervisors decided not to pursue publication then it seems likely that it does not match these criteria either. Note, this isn't a qualitative judgement about your thesis - as it says as much about academia as it does your thesis.
> Submissions that do not contain original or substantive research
Most, if not all master's thesis contain original research, and that is definitely my case as well.
> undergraduate research, course projects, and research proposals, news, or information about political causes
My thesis can't be classified under any of these categories, either.
It does sound like I would be able to publish it there, but I'm going to have to reach out to my supervisors for an endorsement in arXiv, thanks for the tip.
I believe they weren't inclined towards publishing it because I did not quite manage to fulfill their expectations with respect to the scope of the thesis. I came across quite a few bumps along the way which hindered my progress. It was still graded well enough, it has a good amount of original content, and I am relatively satisfied with its quality, though, so it's not like it was a complete disaster.
You should do it, but contact your advisor first. I know I'd like to upload my old masters thesis online, but the university retains a copyright on it. Its possible yours does to. I'm sure the information on that would be available on your schools website in the thesis format/guidelines sections.
Didn't mean to imply it was a disaster; but without being familiar with your work the main signal in your comment was that your supervisors didn't push for publishing it, which might be a hint it's not a good fit for arXiv either. But if your supervisors will endorse you then by all means go for it!
Submissions that do not contain original or substantive research, including undergraduate research, course projects, and research proposals, news, or information about political causes (even those with potential special interest to the academic community) may be declined." as explained in https://arxiv.org/help/moderation
If your supervisors decided not to pursue publication then it seems likely that it does not match these criteria either. Note, this isn't a qualitative judgement about your thesis - as it says as much about academia as it does your thesis.
Additionally, you might run into the endorsement system if you're new to arXiv. https://arxiv.org/help/endorsement