Why is the anime 'Axis Powers Hetalia' rated mature?

Hetalia: Axis Powers (2009– ) .... The entire premise of the show is to portray historical events and stereotypes of countries, so it's really not for those people easily offended by racial stereotyping. Edit. In assumed reference to the Holocaust, ... Suggested rating R for language, some sexual references and brief nudity. Edit. read more

Hetalia is popular because while it is a historical anime which was originally aimed at adult male audiences (seinen) it has lovable characters and similar storylines and canon “relationships” as typical Shonen ai/BL/Yaoi anime even non yaoi anime like Yuri on ice,Haikyuu,Tokyo Ghoul,Fruits Basket etc. read more

If you mean why FUNimation is releasing it as TV-MA (I believe it was originally T, or that's what myanimelist has it as), it's because they thought a few of the visuals were a bit much for younger audiences, and they don't change things unless it's absolutely necessary, and usually only for culture reasons. read more