The government sets an agenda and uses all kinds of tricks to get it through the parliament as fast as possible. A real democratic process is not desired. MPs usually don't have enough time to even read bills, doing proper research is completely out of the question, and therefore they trust the party position. Disagreeing with the party is sanctioned, e.g. you won't get listed in the next election (-> you lose the income you need for your mortgage), less speaking time, etc.

Source: Members of the Bundestag and "Von Rettern und Rebellen" by Klaus-Peter Willsch