The rise of Lord Voldemort has been explained by his charisma, magical power, and desire for control. While the support of a few powerful members of the establishment, i.e. Lucius Malfoy, would aid in a taking over the Ministry of Magic, it is insufficient for total success. Popular appeal is necessary. [NB: Seemingly in the Battle of Hogwarts there were equal numbers on both sides of the fight, so some amount of popular appeal must have been gained.] We can discover a long-term hidden economic catastrophe in the background of the Potterverse which hints to the confounding popular appeal of Lord Voldemort. Let us take a detour through a wizarding census in order to get a sense of the demographic and economic situation of the Potterverse.
Recently a devoted fan came up with an explanation for a discrepancy in the number of students at Hogwarts. With an approximate census of 1,000 students at Hogwarts in the typical year we can estimate the number of wizards in the Potterverse because this is the only Ministry-sanctioned wizarding school in the whole of the United Kingdom. Thus there are approximately 1,000 young witches and wizards between the ages of 11 and 18 in the typical year. Let us consider the year 2011 as it is the most recent year for a UK census and assuming that demographics of the wizarding world are similar to our muggle world, the 1,000 students correspond to roughly 10% of the total population. Therefore we can estimate there are approximately 10,000 (total) witches and wizards in the UK in 2011. With a UK population of approximately 63.2 million (compared to global population of 6.987 billion), this corresponds to roughly 1,100,000 wizards in the world in 2011. This fits within other estimates of 338,100 to 1,127,000 for a census of witches and wizards [NB: The estimate in the prior link was updated to census numbers for 2011 from 2001.]
Based on the student population, the fact that all four houses existed from the founding of the school, and the Chamber of Secrets was created under the foundation of Hogwarts Castle by Salazar Slytherin and not discovered by the broader public until 1993 (which means the castle foundation was never expanded), we know that Hogwarts was built to house at least the 1,000 students currently enrolled. But Hogwarts was founded in 990CE. The population in England at the time was roughly 1.5 million. To consider the entire United Kingdom, we will increase our estimate to 2 million muggles. Assuming similar population statistics for the wizarding and muggle world as we have in the current era, then there are approximately 316(!) witches and wizards in all of the UK. Only a fraction would be school aged, and especially at the founding more witches and wizards would opt to educate their children the traditional way — homeschooling. This means the class sizes would be even further reduced. It would be extremely odd planning to build Hogwarts Castle with four distinct houses for such a small number of students. Additionally, Salazar Slytherin’sdisdain for muggle-born wizards becomes the sign of an obsessed mind — either the muggle-born students are nonexistent at the founding of the school or they are a significant fraction of the student population. As a comparison for how muggle schools function, the University of Oxford (founded in 1096) has had an increase in number of colleges since its founding to accommodate increased enrollment.
This discrepancy can be explained if the amount of magic in the world has been decreasing over time, as in Middle Earth. So in the eras of our Hogwarts founders: Godric Gryffindor, Helga Hufflepuff, Rowena Ravenclaw, and Salazar Slytherin, the fraction of the population that is magical would be greater than at present. Not only may there have been more witches and wizards, but those that were alive may have even been more powerful on the whole. This also explains a related problem: how is Hogwarts Castle so advanced when advances in magic must have been possible in the subsequent 1000 years after its founding.
With the knowledge that the amount of magic in the Potterverse is decreasing — both from a smaller magical population and the weakening overall magical power — we can conclude that the wizarding economy has been in decades or even centuries long recession. The smaller magical population, if it gets too small, would decrease specialization and thus mandate that witches and wizards work in fields in which they are less productive. Perhaps even more importantly, those witches and wizards would be less productive because they would need to do the same work but with less power. This clearly points to a (slow-rolling) economic catastrophe. Over time the magical populace would surely notice that life used to be better — wages were higher and leisure time was easier to come by.
In the muggle world, during extended periods of economic turbulence, authoritarian tendencies may become pronounced in the general populace. This leads to support for demagogues and strong(wo)man leaders who promise to return society to former strength and protect the status quo. Demagogues will also stoke the fear of minorities as scapegoats as the cause of loss of greatness. Lord Voldemort would be just such a leader — rallying a segment of the population that is nostalgic for a more powerful (literally and economically) wizarding past through the scapegoating of the muggle-born minority. Thereby explaining why a leader who is a symbol of hatred and evil can gain so much power.