I'm not Diablo II's biggest fan (in fact, I didn't like the game), but to say it isn't an RPG is simply wrong. Let's take a look at Wiki's summary
The player assumes the role of a hero, fighting monsters while traversing over land and through dungeons. The storyline of Diablo II is played through four acts. Each act follows a predetermined path with preselected quests, although some quests are optional. Each act culminates with the destruction of a boss monster, upon which the player proceeds to the next act. Battle is conducted in real-time from an isometric viewpoint. Players fight monsters to level their character up and gain better items.
Diablo II emphasizes combat, and randomly generates many monster properties, level lay-outs and item drops. Most of the maps themselves are randomly generated. In single player mode, the map is randomly generated but locks the setting thereafter; in multiplayer mode, it resets every time the game is restarted.
Diablo II allows the player to choose between five different character classes: Necromancer, Amazon, Barbarian, Sorceress, and Paladin. Each character has different strengths and weaknesses and sets of skills to choose from, as well as varying beginning attributes.
How's that not fitting in RPG's definition?
Zelda, on the other hand, while it has some RPG elements, is at its core an action-adventure game, there's no way to improve your characters stats (I don't mean life bar, but stats like ATK, DEF, SPD, LUK), you don't gain EXP from fighting enemies, etc.
