Because it became /r/The_Donald's de facto mascot and /r/The_Donald are all alt-right neo-nazis of course and the ADL classified it as antisemitic. Ignoring the very disclaimer they add that "not all Pepe images are symbols of hate speech".

Nevermind that outside of political circles and people who take The Internet way too seriously, Pepe is just a stupid meme that has transformed from "Sad Frog" to "Woznak's friend" to "Betrayer of Woznak" to "Smug Frog/Pepe" to "Rare Pepe". And due to its prevalence in Chan Culture became a stupid meme to tell Channers from Normies (REEEEEEE) with the red Pepe with the wavy tongue. Honestly that meme has transformed quite a bit over the years.

I'm convinced anyone who thinks Pepe is a symbol of the alt-right has no idea that The Internet has its own cultures and subcultures. It just so happens that 4chan and /r/The_Donald have a huge cross-section of users. No surprise that many 4chan memes are prominent there.

The worst defense I've seen are images of Pepe shopped to be Hitler/a Nazi. FFS, you can find the same for Mickey Mouse and Hello Kitty - or nearly anything actually. Shopping something into Hitler is common.

Makes me think if /r/The_Donald chose a Gay Pride or LGBT Flag as their "official flag" if it would cause people to second guess if someone supports LGBT or is an alt-right neo-nazi sympathizer. Talk about extremism.

E:

Just noticed your name and I feel you weren't asking your question seriously, but have a serious answer anyway, Mr. Jimmy Rustles

My nickname suggests that I am a 4chinner (which is true), but the question above was legitimate and was with the best intention.

The moment that Hillary went nuclear with the stupid, harmless frog was the moment that I questioned the things "progressive" people label people who don't agree with them. As silly as it was, it was truly the turning point of me re-examining what I read and listen to with a more skeptical approach.

Thanks for the answer - it really unrustled my jimmies.