If you're arguing from simplicity, Mandarin actually has very simple grammar - much simpler than any of the other languages you mention (no gender, no inflections, no declensions at all), and learning to speak it is not as difficult as people seem to think. Learning their ridiculous writing system (which is morphosyllabic, not ideogram-based) is another question.

The reason Mandarin is not an international language and English is, is because learning English as your L2 gives you the ability to speak to people in the United States and England and Australia and New Zealand and India and lot more besides, where learning Mandarin gives you the ability to speak to people in ... China (and even then, historically, only one small part of China).