Is it because Japanese pronunciation is easier to pick up than Mandarin? Also, is there a such thing as “Chinese” anime (as in made in China / Taiwan being originally in Mandarin), they do have an equvalent of Manga known as Manhua (漫畫) but is it even popular to the same extent as the Japanese counterparts? Why do those types of foreigners favor Japanese (language) over Mandarin regarding pronunciation?
Are sentences in Japanese easier to pronounce in comparison to Mandarin? JP does not have tones (rather pitch accent) with an “alphabet” of some sort (known as ひらがな & カタカナ) even though they have 漢字 (Furigana is used for placing kana above Kanji for native speakers to understand) while Mandarin places a big emphasis on tones and its entirelty written in 漢字 (for transliteration, either Pinyin or Bopomofo is used).
I don’t think they’re looking for the easiest language. Learning any language is hard. There must be a motive to take on such a task.
Weebs love Japanese anime so they want to learn the language. I doubt many people are into Chinese anime or manhua aside from native speakers.