To be able to submit to arXiv, one needs to be endorsed by someone in the domain, which could be a potential hurdle for someone not in academia. (and I'm not about to suggest viXra as a way to get around that)
Yeah, I actually used to be a math post-graduate and have a draft of a paper with an interesting result, so I wanted to publish it but I had no idea how to get "endorsed". This was like 10 years ago and I switched to a career in programming so I doubt I'll even understand my own proof now.