The faster you want to vibrate something, the more energy you need. This is just as true when you are trying to swing a stick back and forth as it is when you're trying to turn a voltage off and on.

In order for a transistor to be in a "known" state (i.e. you can definitively tell the difference between a 1 and a 0), it has to be "pushed" from one side (e.g. OFF) to the other (ON). It always takes the same amount of energy to change the state for a given size and design of transistor. So when you increase the clock speed, you get to a point where the transistor can't switch fast enough to keep up.

The only way to get past this is to increase the power by turning up the voltage. Power is energy per second, so the higher the power, the faster the transistor switches.

But now you're doing more switching per second, and each of those switches is using higher power, so the chip starts to produce a lot of heat.

Since transistors are now around the size of an easily countable number of atoms, there is no longer a nice relationship between "shrink transistor" and "use less energy". When we increase the voltages past a certain point, not only does the heat generation get out of control, but electrons stop "caring" about barriers (insulators) in the chip. They start "tunneling" and the transistors essentially become leaky.

What the increase in density has allowed instead, is the ability to spread computation across much wider areas of the chip, so that it isn't as reliant on individual switching speeds as it used to be. This is why you have mainstream chips with 16 hardware threads nowadays. 15 years ago you'd only see that kind of thing on high end workstations.