The reason is pretty simple, no obscure nuclear physics mysteries here. The nuclei of atoms are made of protons (charge +1) and neutrons (charge 0), with the negative particles (electrons, charge -1) out of the nuclei, in orbitals around.
Because of this, the nuclei of all atoms are strongly positive (charge +n). Like charges repel. So "nude nuclei" strongly repel themselves by electrostatic repulsion. If you can't bring together the nuclei, they're not going to fuse. That's all. Not rocket science, is it?
As a result, all of these "cold fusion" stories out there are always wrong. There's always something that was not taken into account, an experimental or calculation error somewhere, and of course it doesn't work.
Why does "hot fusion" work? It's also easy: because heat means atomic movement, so at high temperatures the nuclei strongly "vibrate". At VERY high temperatures ("thermonuclear temperatures", millions of degrees) these nuclei vibrate so strongly and move so fast that their kinetic energy overcomes the electrostatic repulsion and makes them crash (or approach very much). At such short distances, different forces apply (like the weak nuclear force) and fusion is possible. That's what happens in a star like our Sun (by massive gravitational attraction/compression), in a H-bomb (the primary A-bomb makes the outrageous-energy trick), and we hope in future nuclear fusion reactors (trick expected via powerful magnetic fields or lasers).
So you can see, it's just a matter of basic physics. Like charges repel, heat is movement, much heat needed to overcome the electrostatic repulsion. It wasn't so difficult, was it?
(And sorry for my English, it's sort of hard trying to explain science in a language you don't use daily)
