This question really requires quantum mechanics for a detailed explanation. But here is a simplified version.
In an insulator electrons are bound strongly to their atomic cores and cannot move freely through the material. Therefore no current can flow.
In a metal, due to the interaction between neighboring atoms, typically one or two electrons per atom are no longer bound to their atomic core, but can freely move through the material. Since there are many atoms, there are a very large number of electrons that can carry current. Electrons move easiest through the material if it is ordered over small distances. Heating the material produces disordered energy. Atoms vibrate more strongly and electrons have a harder time moving through the metal without colliding with the cores and losing energy. So the resistance increases with temperature.
Semiconductors are insulators at low temperature. There are no electron
that are not bound to their cores. But the electrons are not bound very
strongly, and as the temperature increases some can absorb enough thermal energy
to become free and able to move through the material. The probability that
an electron gains enough energy to leave its atomic core increases with the
amount of thermal energy present, and the more electrons there are, the more
current can flow for a given voltage. The resistance decreases with
temperature because the number of charge carrier increases. This effect is
much bigger that the small increase in the resistance due to the increased
disorder in the material.