I'll try my best. I'm not an inorganic chemist, but I did take a class on it once, and I'll try to remember as much from it as I can.

Metals are metallic mainly because they're able to form these vast networks of bonds between many atoms. All the properties that we associate with metals - shiny, conducts electricity and heat, such as that - arise because of how this network of bonds interacts with the environment. For instance, having a network means electrons can go from one side of the network to the other which we observe as being electrically conductive.

In other words, the metallicity of an object is an emergent property that's dependent on how large this network of bonds is and what types of bonds make up the network. As a side note, because we know that the network is the basis of metallicity, we can kind of cheat the system by making networks out of otherwise non-metallic objects, and if we make these networks act similarly to the networks found in metals, we get a non-metal that looks and acts like a metal (what we would then call a semiconductor)

It turns out, the higher energy orbitals that you find on heavier atoms have the tendency to form networks. I don't fully remember why, but I think it has something to do with the fact that when you get so heavy, you have so many orbitals that you can just form a ton of bonds at once (I was surprised to learn that metal atoms can form more than 3 bonds with another metal atom)

So basically the lighter elements tend to be non-metals because they don't have the number of orbitals to form a cohesive network (outside of select cases) and the heavier elements tend to be metals because they have so many orbitals that they kind of have to form networks.