Crowns are very inconvenient headgear, they are mighty heavy, made of metal, and they are monstruously expensive, so not only you have to suffer wearing them, but you also have to be extremely careful in so doing, for if the thing falls it will be very badly damaged. This damage will be not only physical, but it will damage the concept of the monarchy itself, think of it as a very bad omen.

In the Middle Ages in the kingdoms of the Iberian Peninsula crowns were used for the solemn occasion of a monarch's coronation, but outside of that, the crown was kept in the treasury of given to custody to someone of the utmost trust. Occasionally it would be buried with a king, as such was the case with Sancho IV the Brave, who was buried with his crown. That gorgeous piece is now in the cathedral of Toledo, where it was unearthed, but it has had some tours in Spain, like when a few years ago it was lent to the monastery of Las Huelgas for an exhubition.

In the realms of the Crown of Aragon, different crowns were used, as it was a composite monarchy, and the king was to be invested with the powers of the different realms separately, which resulted in the king having to tour the realms in a coronation tour, very well and succintly explained by Ramon Muntaner, chapter 29 of his chronicle:

King Peter went to Zaragoza and there he gathered his parliament, and he was crowned with the crown of the kingdom of Aragon, with great solemnity, and great joy, and great feasting. And as his coronation was done, he came to the city of Valencia, and the gathering of the parliament was also great, and he received the crown of the kingdom of Valencia.

And afterwards he went to the city of Barcelona, where he had a great gathering of the parliament. And he received with great glory, and great rejoicing the garland with which he was crowned Count of Barcelona and lord of all of Catalonia.

The difference in the usage of crowns for the kingdoms and a garland for the title of Count of Barcelona is peculiar, but not outside of logic, as it would be unthinkable that a sovereign count could use the same sign of dignity as a king. It is unclear, though whether "garland" has to be understood as some sort of wreath or more likely a bejeweled diadem lacking fleurons or other upwards projecting elements such as the castles one can see ornating the "crown of Sancho IV" (I put it like that because even if Sancho used it, it predates him by about 100 years).

The coronation was an occasion of great solemnity, and not only the king was receiving the dignity of kingship, but also the lords were paying homage to him. This part of legitimisation by the consent of the vassals was particularly important for pretenders with dubious claims to legitimacy, like king Henry II of Castile, who was formally crowned in the city of Burgos, where received the homage due to an overlord. Chancellor Pero López de Ayala, who had served king Peter I, and later defected to Peter's bastard half-brother's side, writes it in a very interesting manner:

And later king Henry had great workings done in the monastery of Las Huelgas, which is a royal monastery of ladies in the city of Burgos which had been founded by the kings of Castile, he was crowned king there, and from here onwards in this chronicle shall him be called king. And after he was crowned, the people of the city of Burgos kissed his hand as king, and so did as well many knights who had come to his side

Coronations eventually fell out of fashion in the christian kingdoms of what today is Spain, as both in the realms of the Crown of Castile and the realms of the Crown of Aragon it was understood that what makes you king is being sworn as such by the Cortes (parliament). This becomes painfully and funnily obvious with the message delivered in the Cortes of 1518, when Charles I (later known as Emperor Charles V) received this message from the president of the Cortes:

Very powerful lord, before any other thing, we want to bring to Your Highness' memory that you were called and chosen as king, the interpretation of which is to rule well, and for any other way it would not be ruling well but to dissipate, you would not be called or named king. [...] So, very poweful lord, if this is true, for Your Highness is to reign, and reigning has the property that when the subjects sleep, the king is awake, so Your Highness has to be be so born, for in reality you are our mercenary, and for this your subjects pay you and give you part of their earnings and fruits

In the realms of Aragon, the king had to be sworn in by the Major Justice of Aragon and in presence of the Cortes Generales, which would recite a very specific formula making it patently clear what is the actual origin of the king's power: We, who are as much as you, and who together are much more than you, make you king of Aragon if you swear to uphold the fueros, and if not, we don't.

The use of crowns was already rare enough in the kingdoms of the Spains, only for coronations, but eventually fell out of fashion entirely in no small part due to the weakening of the Crown's power in the Late Middle Ages. Crowns were still featured in coins, and ocasionally in paintings, but this is purely symbolic, to represent royalty. The presence of crowns on numismatic is quite interesting in and of itself, as one can very well see the skilled hand of the die creators, but also some artistic trends, like the very classicist coins from Milan in the time of Philip III, where you can see the king donning a radiate crown.