Ultrabooks are about thin and light. Higher clock speed cpus generally run hotter and take more power. These more powerful cpus drain batteries faster and require a thicker heatsink.
If software can use all the cpu cores, this lower speed is less of an issue.
Also note that clock speed is not always an indicator of performance. Look at how the 2.4 GHz Core 2 duo cpus killed 3.8 GHz P4s(including the dual core ones).
Intels current cpus have a trick too. when running a program that only uses one core, the others can enter a low power state and allow the single core to clock up to a higher speed(Turbo).