This is not entirely correct. The writings on the turtle shells (oracle bones) were indeed blocky and pictorial, but not called zhuanshu.

Zhuanshu was developed under Qin Shihuangdi (with help from Li Si if I'm correct) and is easily recognised by the curved corners and overall 'round' look. This (quite radical) script reform done by the Qin took place about 1000 years after the oracle bones writings.

Because of the invention of paper early in the first century AD writing became more common. The seal script however was very time comsuming to use, so during the Han dynasty a new script was developed, called 'clerk script' (Kaishu).

Eventhough is has of course developed, kaishu has remained pretty much the same and is still used in today's caligraphy. Nowadays zhuanshu seems to be used for seals only.