High(er) Frequency means that you have to run at a higher core voltage (this is, in part, because gate propagation delay is inversely proportional to bias voltage, i.e., the voltage corresponding to a 'high' or 1 bit). You have to decrease the propagation delay so that the clock tick gets everywhere in the processor quickly (one part of the clock doesn't lag the other parts, called clock skew). So in order for things to run faster, you'd have to run it at a higher V(bias). Now that means that there are higher thermal costs (things get hotter) - heat produced is proportion to voltage.

So its now mostly a thermal management problem. This is the primary problem in the newer chips. Even though we can pack in more transistors, we can't get signals among them faster without higher V and making it run too hot.

Therefore, our solution is to use the extra transistors to create a separate new processor, running with a different clock, so the 'tick' doesn't have to reach all parts of this rather big chip, but just needs to be synchronized intra-core.