"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.

Additionally, you might run into the endorsement system if you're new to arXiv. https://arxiv.org/help/endorsement