It's because Japanese manga and animation are new and interesting to people overseas, I think. The stories are simple, and the characters' faces clearly express their feelings. The unique designs of their cloths may be one of the reason.
Nowadays many foreign bookstores sell manga, and TV stations broadcasts Japanese animation. As the opportunity to see them surely increased, it's natural to be popular.
64 words
日本のアニメの多くは、キャラクターのデザインが可愛い。
ストーリーは、王道、テンプレ、ハーレム等、視聴者が楽しめる要素を取り入れています。