[Moppie]

A 4 valve (or more) per cylinder engine is much simpler to design and build if you have one cam shaft for inlet and one for exhaust valves per bank of cylinders.
It also allows a less cluttered cylinder head, and more flexiablity with head and compustion chamber design and gives more freedom for spark plug placement.


If you have a search for DOHC vs SOHC you will find lots of other threads that go into a lot more detail, and contain far more accurate answers than I can give.