It does do a fair amount of filtering of submissions, and it's a long term archive (e.g. for the next 100+ years). I suspect both (but with the former dominating) are the issue.

Well, technically, it can also compile your tex file if you upload the tex file instead of the pdf directly, which helps a lot in standardizing the stylistic structure between preprints. Most other repositories are wild west and inconsistent. I really appreciate the similarity in style applied to most preprints there. Moreover, this means you can also download not just the pdf, but the source tex file to, which can be very useful.

Also the sources and has a very tame but useful pre-acceptance process.

Technically yes, socially no.