There is nothing wrong with the game play of PoE beyond desync issues, issues D3 have as well, the AI does exactly what it's supposed to, the graphics is subjective as far as style goes, the actual quality of it is as good as any other game in the genre.
Because their competitors can...
I'm sure both will be around for years, they are designed for very polar audiences, PoE more for the oldschool ARPG hardcore player and D3 more for the younger insta gratification MMO minded player.