ARod used PEDs over and over. They say he tested positive from 2001 - 2003 and there's evidence he used them from 2010 - 2012 too. MLB has finally had enough it looks like. With him being the highest-paid player, recruiting other players for Biogenesis and destroying evidence from the clinic, it's a big black eye for the league. So they're using him as an example to other athletes who think they can get away with it.
It's the longest non-lifetime ban in baseball history and the longest handed down under the Joint Drug Agreement (by far). There are 12 other players involved in the investigation but he went much further to cover it up. I think he deserves the 211 games.