I'm guessing it's because spider monkys have long curling tails that look a bit like an @ if you squint: http://kids.nationalgeographic.com/content/dam/kids/photos/a...

The circle bit looks like a monkey tail, simple as that. And there was no need for a shorter word, as it really wasn't used in common speech or writing (the French "à" was/is used in the commercial sense).

My guess used to be that it started as a "Klammer-A" ("a in a bracket") and that at some point the joking extension from A to Affe (monkey or ape, everyday German does not make a distinction) went viral, supported by the tail-like shape. Now that I know that the monkey connection even exists in languages without the a -> Affe link I'm not so confident anymore.

I can only speculate but I believe it is due to the fact that it kind of looks like a paper clip shaped like a monkey ear (speculation on my part but I've heard that before).

Klammer is short for Büroklammer (= paper clip) and Affe is monkey (or ape).

The at symbol looks like it has a long tail wrapping around over its head, like a spider monkey sometimes sits.