If you want a bit of srs deconstruction:
Slenderman has captured the imagination because he fufills many tropes of 'creepy', and his origin as a 'creation of the internet' has raised his popularity even further.
He is a permanent resident of the uncanny valley. He wears clothes and is humanoid in appearance and yet he is not human. The exact things which make him non-human are specifially tailored to psychological horror.
He is the unknown.
He doesn't move, he has no face, he's tall and skinny and in the original version has wisps instead of proper arms. He's basically a ghost or boogyman, but unlock those two he has not been in the cultural eye long enough to have all the extra 'less scary' baggage(cartoony 'friendly' ghosts or laughably unscary renditions) that these other 'monsters' have picked up over time.
Having no face means he has no readable emotions, no expression. You cannot understand him, you cannot relate to him. There's a lot of power in the face, in terms of emotional impact.
Think for a serious moment about a real life encounter with someone/some 'thing' with out a face at all, just completely blank. Be honest now, it's pretty damn unsettling right?
A zombie sits pretty much at the very pit of the uncanny valley. Zombies are so popular cause they're so human, and yet twisted into this frighteningly 'unhuman' thing. Slenderman arguably achieves this effect to even greater extent, possibly for the very reason that he is so 'alien', and yet he's right there wearing a suit like people do, looking almost like a human.
People like us we can understand, evil murders and beastial monsters we can understand. Even zombies we can understand. This 'thing' we cannot, for it is so alien, the opposite of animals or beasts, more like an 'object' than a living thing in how 'detached' it is, and yet with an intelligence or driving force there which leads it to do these things.
People are into that. A revenant or wrath like figure, who will stalk you and there is no way to escape, no way to confront. He is merely there, waiting to do creepy stuff when you're not looking.
Again, people are into that.
It also plays off countour bias. Big, rounded shapes are soothing. Sharp, thin shapes are dangerous and unsettling. It's an instinctive thing. He is unnaturally tall and quite skinny. Those tendrils that you often see coming from him are like long spikes. Contour bias again.
and finally, it picked up in popularity around the right dens of the internet to see it's exposure rise and people get into him. For example this recent resurgence is owed to that slender game.
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Ok so that wasn't really a proper deconstruction, but it did throw in a few relevant buzz words here and there. That counts right?
