Minecraft is a sandbox voxel game.
There are multiple mechanics to the game that add to the player experience, but the most likely reason as to Minecraft's vast number of players and fame is the fact that everything (If you don't use mods) is a cube. Mojang is also consistently adding features without breaking things that players are used to.
Minecraft is also very kid-friendly on its own. Players in a single-player world or among friends will not be exposed to bad language or inappropriate content that they aren't already exposed to.
Some features that players such as myself are very happy about are Redstone (A non-real form of electricity. Redstone mechanics differ in Java and Bedrock editions), Commands (The ability to change the world around you without having to move your character. Commands and uses differ in Java and Bedrock editions), Falling blocks (When cubes taken out of reality, like sand and gravel fall if nothing is beneath them), various mobiles (also called mobs, are interactive beings in the game that can be neutral, aggressive or both), MultiPlayer (The ability to play with other players either locally or globally), etc.
