NOTE: This is my theory on why it is popular. If you think otherwise, comment it!

MY THEORY: I would say Minecraft is so popular because it is so unique. The pixels are usually in 8-bit games, yet this is 3-D. Garry's Mod is 3-D as well, but it is harder to build stuff, and not pixelated. How many 3-D games do you see that are blocky? The blocks also contribute to why it is easy to build. You can use math and multiplication to figure out what goes where. Now moving onto the experience. You have to survive and thrive. A survival game! You can build so many things with infinite opportunities as well as in creative. Go on servers! Explore the land with others while crafting awesome tools with the resources you collected! It's basically gradual progress building up. Bored? Install mods! Its kinda hard to get tired of the game since there are so many things to do! So unique, yet, so simple...