Slop.

Short answer: Francisco Franco and continuing trends during capitalism

Long answer: Under fascism, Spain developed an extremely centralized model, in which a few big cities (Madrid, Barcelona, Sevilla, Valencia, Bilbao, Donosti, Málaga and a few others) get all the economic activity. The roads, railways, hospitals, schools, universities, etc, were built to service preferentially these big cities, in particular Madrid (the service and political capital) with the highway network of the country being radially developed around Madrid, highways A1 to A6 connecting Madrid with 6 ends of the Iberian Peninsula.

This trend, which originally came as a centrally planned choice during fascism, has been maintained through inaction and inertia during capitalism, and it has led to the massive populating of Madrid and its surroundings turning it into a metropolitan area of 6+mn inhabitants in a country with just 47mn. The result is that business only opens up in Madrid and a few other privileged cities around the coast of Spain, leasing the doughnut inbetween to be a massively depopulated area, comparable to e.g. arctic areas of Scandinavian countries, while Madrid itself bloats. People continuously migrate to Madrid for the job opportunities at rates not supported by housing construction projects especially since the 2008 bubble burst. The result is the gentrification of the vast majority of Madrid and its periphery, with housing and rent prices skyrocketing, in my area about +13% only in the last year.

The lack of state investment in infrastructure or outright job creation in the so-called "España vaciada", or "emptied-out Spain", leads to people being essentially forced to become economic migrants within their own country and to all life together in increasingly sardine-can apartments as housing prices go up and people are forced into being roommates at 30+ years of age. Right now the situation is such that a secondary school teacher in Madrid makes about 1800€ a month, and rent prices are around 25€/m², so that a meager 40m² flat, which is smaller than many ofnthe the 1950s khruschyovki built in the USSR, will cost a teacher more than half their salary.

Centralization in and out of itself isn't necessarily a bad thing, it can be quite an efficient tool both economically and environmentally. Flats and densely urbanized areas are really much better than suburban sprawl, and having a hospital and a university cover hundreds of thousands of people has a lot of advantages. But Madrid is overdone, and there are a lot of interesting provinces completely hollowed out of their population just because business is by design more efficient in Madrid. There could be many more smaller densely packed cities all over Castilla y León and Castilla-La Mancha with ~500k inhabitants which would revitalize the regions and maintain a lot of the culture that's being lost to the hollowing out of the regions.