The Revolutions of 1848 are sometimes called "the turning point in history that did not turn." Why is this so?

The first thing to note, when discussing the Revolutions of 1848, is that this moment in European history did not emerge out of a vacuum. Rather, these events were influenced by a much larger context, shaped in the aftermath of the French Revolution.
The French Revolution's effects were radical, but for the purposes of this answer, it's important to note that, ultimately, with the defeat of Napoleon, it ended in failure. The traditionalists won that struggle, and in their victory, they reimposed much of the pre-Revolutionary status-quo. In doing so, they sought to suppress as many of those radical threads unleashed by the Revolution as they could hope to manage.
Be aware then that, while 1848 was the greatest and most intense manifestation of these energies, it was not entirely unique. One can easily point towards the Revolutions of 1830, for example, as another significant expression of the same revolutionary fervor. From this respect, when we speak about the Revolution of 1848, we need to recognize that it exists as part of a much larger history, shaped in the legacy of the French Revolution.
When seen from a purely revolutionary perspective, the Year of Revolutions 1848 ended in failure. In France, observe how the monarchy was overthrown as the Second Republic took shape. However, the Second Republic would be gripped by deep social and political divisions. The elections would carry Louis Napoleon Bonaparte to power, who would lead the transition from Republic back to Empire.
This sense of reversal would be mirrored in the other countries where the Revolutions of 1848 spread. Nationalist and Liberal revolutionary movements in Italy, Germany, and Central Europe were ultimately defeated by counter-revolutionary forces. Even so, this remains a critical moment in the history of Europe, both in terms of the transition towards democracy as well as towards eventual nationalistic unification. Those long-term implications are immeasurably important, but the fact remains that, in the immediate aftermath, those Revolutionary impulses remained unfulfilled, and the struggle to achieve those ideals was still yet to be achieved.

The Revolutions of 1848 were political rebellions and upheavals across Europe. These revolutionary movements and protests were very similar to the Arab Spring that swept across the Middle East in 2011 and advocated democratic reforms. Both movements utilized both violent and nonviolent protests, were violently suppressed by those in power, and challenged the traditional rulers and government structures. In addition, both revolutionary movements were unable to achieve their reforms and goals.
In the Revolutions of 1848, a combination of middle- and lower-class workers and reformers called for democratic reforms such as freedom of speech, freedom of the press, increased participation in government and suffrage, and increased government involvement in the economy. Thus, much of these reform ideas stemmed from the advent of the Industrial Revolution, which lead to increased gaps between the rich and poor and oppressive working conditions for the laboring classes who worked in factories and cities. However, in most European countries, these demands for reforms were met with limited success. Monarchies and aristocratic governments suppressed the movements, often violently, and many reformers were forced into exile, seeking refuge in the United States.