I think that a big part of his appeal is with the generation of comedy he grew up in, and the longevity of his career.

As a 50 year old, he started his career in the 80s and 90s. Some of the most prolific comedians of that era included, but are not limited to, Sam Kinison, George Carlin, and Eddie Murphy, all of whom were well known to have a lack of filter and an excess of vulgarity and lewdness. This paved the way for comics like Rob Schimmel, Dave Attel, and of course Louis CK, all of whom had that gritty unfiltered and vulgar touch to their comedy, Schimmel even being rated X for how vulgar his set was.

Unlike Attel who ebbed in and out of obscurity, CK persisted, but unlike his comedic mentors like Carlin, maintained a predominantly lewd and unsettling approach, instead of developing a more nuanced view of what is 'funny'. On top of that, he could get away with it because vapid and vulgar comedy was on a meteoric rise, comedy central breaking down many censorship barriers that people before had struggled with. The internet only accelerated this further I feel.

As time went on, comedy and culture would see a rise of political correctness. Realistically I just think CK just found a really successful groove that was very lewd observational comedy, and he let it become his actual personality and character. It sucks because it gives the impression he became what was funny - and unfortunately what was funny was thinking his comedy was satire making light of otherwise really fucked-up thoughts such as public masturbation.

I think the final nail in the coffin though was discussion between victims, more importantly across social media where people who would otherwise not be able to communicate could - like via facebook or twitter.

I hope that with time, we can identify why men like Louis developed into people that feel compelled to do these things. Maybe it is a fame or a power thing, but I can't brush off the feeling that the behavior is a symptom of their craft, and not vice versa.

TL;DR

Louis grew to fame during an era where vulgar and unfiltered comedy was blowing up. He got too caught up in the ideas and let it bleed into his every day life. He is now the brunt of his jokes, and it's not funny.