Like, seeing it in original context, it neither screams "Nazi" nor "having enough aesthetic value for Nazis to bother appropriating"
Pepe was a 4chan meme for a long time before it went mainstream in 2014-2015. Like another user said, a big part of it was that these people who had been using the meme for years turned into nazis and took Pepe with them. But, another part of it is that some 4chan types got really angry that "normies" co-opted their beloved Pepe. So they started cranking out the most vile bigoted Pepe memes they could, some ironic and some unironic, to scare away the normies. The /pol/ types liked these and started using them unironically, spreading them beyond 4chan.
They did the same thing when Hot Topic started selling the rage guy. They said he was actually the "race guy" and started posting racist stuff and then publicly demanded to know why Hot Topic would dare to sell racist paraphernalia.
Hot Topic was basically like "okay, whatever, we know this is a prank, but we're just gonna pull it because it's not worth it"
That probably served as the template for nazi pepe memes.
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.
Same reason everyone around here likes piggy poop balls. There's nothing inherent about it, It just happened one day and they went on with it for the meme value
Here's a TL;DR timeline
Pepe is already prevalent 4Chan culture
Pepe begins to broaden it's horizons and is adopted into general meme culture
Pepe is adopted in many ways, including the innocuous and the racist
2016 election is afoot, online trolling enters the discourse, 4Chan is thrusted into the hotseat
Various anti-hate organizations associated closely with Hillary Clinton see Pepe and the 👌 symbol and classify it as white suprem. symbols
4Chan decides to own it up and start making explicitly racist memes and materials after the media hit, and the rest is history
-7DeadlyFetishes
What's not to get? They liked pepe, kek is a corruption of lol and then they found a frog god who just happened to be named kek.
They have no culture of their own so they need to constantly absorb and distort other people's shit lol
