Like others already stated is mainly due to the miniaturization of the hardware.

A computer on its lowest layer consists of logic gates. These are 'blocks' of switches that can, for example, compare input signals and compute simple logic like AND, OR, etc. Those switches need to be able to be activated or deactivated by other logic to make a "computer".

At the beginning those where e.g. electrical relays. The first electrical switch, was as far as I know the vacuum tube. Those things were quite big.

Then came the transistor, which could be switched on by a small voltage.

Then came the integrated monolithic computer chip. Which allowed to put complex combinations of those transistors very densly packed onto one chip.

So this process reduced a thing that can be held in a hand to a microscopic 5nm integrated circuit on a silicon waver, which also reduced the energy required for computations considerably.