He was so obnoxious that I guess it is easy to be happy about that, but he was so small time in the grand scheme of things. But nearly no bankers responsible for the 2008 financial crisis ever went to jail: https://www.theatlantic.com/magazine/archive/2015/09/how-wal...
Here's the facts as I understand them:
1) He ran a hedge fund, and lost a lot of money.
2) He didn't disclose to his investors that he lost the money, and falsified reports.
3) In an effort to make sure people didn't lose money (whether to protect himself or to make sure his investors were made whole) he took profits from another company he ran and used them to pay these investors.
4) Almost all investors in the hedge fund ended up making a profit because of this.
It's fairly easy to interpret this as someone that screwed up bad, and then broke the law to cover it up and try to make it right, and by lucky happenstance was actually able to get people's money back, but still ended up breaking the law. That's a fairly good story to have, and I would think lends itself towards a mitigated sentence.
On the other hand, he repeatedly presented himself in an unsympathetic way that antagonized the judge and the regulatory bodies involved.
It is, to put it mildly, a fairly complex and (to my layman eyes) unique situation, compounded by the fact that his behavior may or may not have to do with being socially inept (is it right to punish someone for a social disability, if that is indeed the case here?)
There was a discussion about this when the guilty verdict was announced[1]. Much of my info comes from that article and this other one[1] from the same time period.
1: https://news.ycombinator.com/item?id=14931004
2: https://www.nytimes.com/2017/08/04/business/dealbook/martin-...