"Well, duh, might as well ask why is a tree good? Why is the sunset good? Why are boobs good?"
In seriousness, it's hard to explain why it's good but Showtime has put out good work in recent years. Check it out and like @mseabolt said, Voight is worth the watch.
It's got a sopranos or breaking bad feel where you're cheering for an asshole. The characters and plots are unique, there is just some good acting by a lot of the characters and the feel/vibe of he show is just different than most shows out there.
I still like the show but I don't know that I'd say this is one of the best seasons. The flashbacks get annoying. This season does have the BB/Sopranos feel where all the bad behavior has long term consequences.