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).
The people in the nebulously defined West didn’t get into Asian comics first and then gravitated towards the Japanese offerings. Japanese manga companies saw a market for expansion and took advantage of it. Manga is Japanese in the minds of consumers. Most people probably aren’t aware of the Chinese or Korean varieties. Interest in the language came after.
Japanese may be easier to pronounce for us atonal alphabetters. I would say Chinese beats Japanese by having simpler grammar. None of these linguistic reasons matter though. There are plenty of people who learn mandarin today but they didn’t get into it through comics. Most Japanese learners don’t do it for business reasons. The stereotypes are different.