To provide a context, 1848 was an important year in European history. An economic depression in the later 1840s coupled with crop failures caused widespread hardships, including famine, across the continent. These hardships led to political uprisings. The German states, which had not yet unified into a single nation, experienced revolts brought on by hunger and the effects of industrialization. 

Worried elites, fearing full-scale socialism would take root, agreed to give citizens more rights, including the opportunity to participate in government. This became part of a movement to unify all of the German principalities into one country. While for a short time it seemed a new, more democratic and inclusive order would prevail and Germany would unite on the basis of enlightened and equalitarian principles, fighting between the various political factions soon undermined that dream. 

One major dispute involved whether to involve Austria in the new German state or exclude it while relying on Prussia for leadership. More importantly, however, popular support for the rebellion began to fade as the common people realized the new liberal government would do no more for them than the old conservative government had. 

Emboldened, the conservatives had some victories. Austria-Hungary used the military to crush several rebellions, and in Prussia Frederick William IV dissolved the legislative assembly. Attempts to establish a unified Germany around a limited monarchy constrained by an elected legislature (the English model) failed. The working classes lost their will and energy to rebel, and the aristocrats, less fearful of being murdered in their beds, pushed for the old status quo. 

In summary, impetus for the kind of revolutionary change that would have brought representative government to Germany failed to achieve sufficient momentum to overcome the entrenched walls of tradition and privilege that had been in place since medieval times. Democracy would have to await another day.

There are three main reasons why the German Revolution of 1848 failed.

1. The people at the Frankfurt Assembly did not really represent the full spectrum of people who wanted change.  The delegates to the assembly were all moderate, middle class liberals.  This meant that they did not represent the poorer, more radical people who had been behind many of the protests against the king.  This weakened the assemblies bargaining position, making the king less likely to accept their proposals.
2. The people at the Frankfurt Assembly were not united.  Most importantly, they disagreed as to what countries should be included in the new, unified, Germany that they were proposing.  This, too, weakened them.
3. By the end of the Frankfurt Assembly, King Frederick William IV of Prussia was in a much stronger position.  He had been weakened by protests in Prussia and had promised to grant a constitution to his people.  By the time the Assembly ended, however, he had used military power to suppress radicals and was feeling more confident.  In addition, he was strengthened because there was some degree of backlash against revolutions in general on the part of many of the people.

These three factors contributed to the failure of the German Revolution of 1848.