Why the frequency of single-core CPUs has stopped increasing?
If you keep raising the clock, eventually you will have power consumption/temperature problems (you probably remember Pentium 4 CPUs which ran fast and hot). You can work around those, of course, but then your cooling system becomes more expensive.

Thus, multi-core (and other approaches, like improving the CPU architecture/organization - so much that comparing CPUs of different families/manufacturers is useless) were born. 

Moreover, modern software is multithreaded and will benefit from multicore more than it does benefit from faster clocks.