The simple analogy I use is the spoon/fork analogy.  Think of a supercomputer as a whole bunch of forks.  You can try feeding yourself soup with 25 forks at once and it will work but it's not efficent.  ASICs are Application Specific Integrated Circuits - basically they were engineered for this one task and one task only which is to hash SHA-256.  The ASIC would be equivalent to a spoon.  One little spoon can feed you soup faster than even 25 forks stuffing your mouth at once.

So ASICs are made solely for this purpose so even though they're clocked lower they do this one job damn well.

The spork would be FPGAs