I think the popularity has massively surprised the developers. I started because my son recommended it and then was instantly hooked. Same for my daughter. She even created a steam account to buy it, so not a regular PC gamer.
Its difficult to put into words why it is addictive, but its just seems to have really well integrated mechanics. You have a basic set of principles in terms of the physics, crafting, combat, etc. but the player has a lot of freedom in how they use those mechanics and it never feels like a grind. Plus, it always seems like you have a project to work on. Whether that is building a base, or establishing an outpost, or setting off on an expedition into dangerous territory to get the materials you need.
Everything is destructible. I just spent some time clearing ground for a base extension, which involved chopping down trees with an axe (which is fun in itself as they actually fall and can land on you if you aren't careful), chopping up the fallen trees, removing the stumps, breaking up any rocks in the area (using a pickaxe), then leveling the ground and clearing grass and shrubs (with a hoe) until finally I have a level piece of clear ground. The wood and stone gathered in the clearing operation can then be used in construction.
The combat is fun and death is meaningful (you lose skills and have to retrieve the body to get your equipment). Also I love games where I can go sneaky archer. It isn't quite as good as Skyrim Requiem in that respect, but it integrates well into the rest of the game.
Finally, the game generates funny stories and very entertaining moments. Everyone dies at least once due to a falling tree for example. For a longer example, my daughter and I had built a raft and sailed to a dangerous area (for our current skills and equipment). We got attacked and had to run to the raft but she didn't make it and died on the shore. I sailed the raft back to our spawn point (you can set any bed that is covered by a roof as a spawn point) to pick her up then we set off to retrieve the body as it had all of her gear. As the raft got near the hostile shore at night we could see the enemies that killed us - various type of grey dwarf. I started shooting them with my bow while stood on the raft, so they jumped in the water and swam toward us. Minor panic ensued, but I managed to sail the raft backwards while shooting them in the water. A couple made it to the edge of the raft and I couldn't hit them with the bow, so I changed to axe and started attacking them. Unfortunately, when you swing an axe it can damage anything in that arc. So while I was hitting the dwarves I was also hitting the raft. The raft suddenly broke apart and we ended up in the water. We swam toward the shore, but you use up stamina when swimming and we didn't have enough. We both drowned just yards from the beach
