Q: Why do you think the Bus speed pre-set?
Answer: There are various devices which can be attached to the bus, unfortunately some of them operate at different speeds, some faster than others. This presents a problem when devices must use the bus. 
Devices which operate slower than others  will not be able to transmit/recieve data faster enough. This means that the clock bus speed must be set so that all devices can communicate on the bus at the same speed.
A major draw back to the clock bus speed is that devices which can work at fast speeds will not be able to work to their full potential as they will be slowed down, when transfering/recieving data via the bus.
Q: Can you think of a solution that would enable faster devices to work at a more productive level?