There are some reasons for this:

• industrialization. Lots of former agricultural regions became industrialized societies. The population grew extensively, and through this, lots of people became very poor (especially in the south of Germany).
• steamboats. Through the popularization of steamboats it became safer and cheaper to cross the ocean.
• good economy in the US. The economy was booming and promised a wealthy life.
• free land in the US. Immigrants had the opportunity of land acquisition - for free.
• marketing. Shipping companies advertised and promised a better, wealthier life - to sell their tickets, obviously. Beside, there are lots of letters of immigrants from the US, that wrote home to Germany and informed the people about the country and the better circumstances. Those letters were often shared among the communities or printed in newsletters, so they had a wide reach.

Here you can find a database (unfortunatelly only in german) of the immigrants and lots of documents like passanger lists.