The popularity of Overlord can be credited to its unconventional story of the apparently evil Ainz and his minions. While we have been force-fed the good versus evil narrative that the majority of stories tend to take, Overlord humanizes the evil that Ainz and the Great Tomb of Nazarick represent.
Though you may not agree with his methods, you are compelled to admire Ainz and his merry band of horrors for their obnoxious strength and fairly relatable motivations. Even if you may sometimes get a bit weary of the way in which they manage to defeat their enemies without breaking into sweat, the style of storytelling and richness of detail give you interesting characters to root for.