It's not that they specifically sought it out to take. It was just such a part of the online meme culture for years before 2015. You had people who spent a lot of their adolescence with pepe as a meme turning into Nazis so it went with them. But even then, it wasn't a specific Nazi thing. What really helped them co-opt it though was liberals freaking out over pepe being a white nationalist symbol. You had a bunch of clueless people who didn't know anything about memes only hearing that it's a nazi frog meme. Then it worked its way up to Chilldawg and that was it.