Religious and economic differences combined with suspicion about Prussia kept the German states divided. The strength of Austria and varied aims of the nationalists also made unification unlikely.
The German states were independent of each other. In the Industrial Revolution they developed at different rates. A division grew between the northern and southern states
The more northerly states were under the influence of Prussia. While the development of the Zollverein in the 1820s and 1830s was most beneficial to Prussia, the agreements with its neighbours to allow free travel of people and goods encouraged the growth and prosperity of all the states involved.
Development was furthered by the introduction of the railways in the 1830s. Rail communication helped end the isolation of the northern states. Better transport of natural resources and goods further benefited economic development.
However, the Zollverein was also a symbol of division:
many states in the south had not joined
Austria had excluded itself
Prussia can be seen to have used the Zollverein to further its own agenda and increase its influence
the economic success of Prussia was envied by many of the German states
Industrialisation was much slower in the southern states, which tended to fall under the influence of Austria.
Although it appears that there was increased economic unity, the Zollverein effectively divided the German states into two camps - those under Prussian influence and those under Austrian.