Why is Graphene So Strong?
If I understood what strong means, then graphene has high strength because of the 2-D layer structure of carbon atoms. The strength can also be attributed to its $sp^{2}$ hybridisation.

Any crystalline material will have a lattice structure. When we mean strength of a material its usually the ability to break apart the bond between two lattice atoms. But the observed strength is usually 2 or 3 orders lower than theoretically predicted strength due to flaws (voids, dislocations etc). But when graphene has a near flawless 2-D structure we can reach its theoretical strength. Though its is a very expensive process to actually realize "flawless". Even with flaws its strength is really high compared to steel.

So its essentially the 2-D one atom layer structure that can be responsible for its high strength.

One important thing is though graphene has high strength and elasticity (ability to get stretched)  it is relatively brittle (fracture toughness ~4 MPa$\sqrt{m}$).