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An easy is answer is that there were more German royal houses because Germany wasn't unified till 1871. There were many different German sovereign states and as a result of that they were often intermarried with the royal houses of Europe.

This has however deeper roots. Why did the royal houses marry each other? With European royalty, noble houses that sit on thrones and their direct relatives, there were a certain set of guidelines for who they could marry. Marriage in itself was quite logically used to further the dynasty by producing children. If your sons and your daughters don't marry, they will have no children and thus when they die there will be a strugle for power and someone else will take over. European dynasties were very focused on the dynastic success and prestige so simply dying out because of a lack of succcessors is something they would try to avoid. This was of course most important for the sons of the dynasty because they would carry on the dynasty name. And in some countries women were even excluded from inheriting so it was vital to have a male successor. If you didn't have enough children or if you only had daughters there was a chance that your dynasty would die out. This happened most famously with the Burgundian Valois dynasty who had a territory that contained the Low Countries (currently Belgium, Luxembourg and The Netherlands) and much of what is now eastern France. Their last duke died with only a daughter surviving him, this daughter was married to a Habsburg which resulted in much of the Burgundian lands going to the Habsburgs. This brings up another reason for marrying other royal families, if you're lucky you can even plant your dynasty on their throne, again the Habsburgs were famous for this as they repeated this trick, they had used to take over the Burgundian land, on the Spanish royal family. And with a stroke of luck and strategic marriages the Habsburgs had taken over the Low Countries and the crowns of Spain without much of a fight in the span of 40 years, which catapulted them from a medium sized power to the greatest royal family in all of Europe and one of the great dynasties of all time.

So, you wanted to secure children for your dynasty and maybe also expand your dynastic lands by marrying your sons and daughters with other royalty. But why would anyone then marry the Habsburgs? "They're only doing it to take over our land, we will be just as well off if we marry one of the local nobles". Well it's not quite that simple as that, there were other reasons for marrying members of other royal houses. A marriage would also bring with it an alliance. If your brother in law was the king of France then you could rely on him to help when you are in need of help, for example in a war but also if your people rise up against you. As an example: when the Orange-Nassau dynasty was overthrown in the Netherlands in the late 18th century. The Stadtholder was married to a Prussian lady whose uncle just happened to be the king of Prussia. This king Frederick William II assembled an army marched to the Netherlands, defeated the rebellion and placed his niece and her husband back in power.

It also brought a certain element of prestige to marry someone of another royal family. This was so much the case that some royal families married their children almost exclusively to partners of equal rank. A princes has to marry a prince. Marrying someone of a lower status was frowned upon as it would lower the prestige of the dynasty and this would weaken the royal bloodline. The royals would like to uphold the belief that royalty had been ordained by god for their role and were thus almost separate beings from the rest of the population. If you married someone from 'the rest of the population' then this argument weakens.

You'll want to marry your children to someone of equal or if possible higher rank and you want it to be to a potential ally. The result of this is that certain marriage blocks would evolve but to understand these there is another factor: religion. Although the importance of religion in dynastic marriages had started to decrease in the late 19th and 20th centuries it was still very important before that. Catholics would marry catholics, protestants would marry other protestants and orthodox christians would marry other orthodox christians. Occasionally these lines were crossed, most often between the orthodox and protestants camps.

If you want to marry your children you have to take into account all of these things. They have to marry someone of equal rank, they have to have the same religion, the woman has to be of childbearing age and if it's possible getting a nice alliance with all of this would be very helpful. In some circumstances they would of course deviate from these rules if necessary, if there was no one available who would meet all the criteria or if an alliance with a more local noble would bring other significant advantages.

Let's look at an example, I'll go with the Habsburgs again as I've referenced them before. The Spanish Habsburgs of the 16th century, our starting point is Charles V/I of Spain as the head of the dynasty. He himself was married to a Portuguese princess, Isabella of Portugal daughter of king Manuel of Portugal of the House of Aviz. This was someone of equal rank (daughter of a king) Portugal was a handy alliance and she was also catholic. With her Charles had three children who survived into adulthood: Philip, Mary and Joanna. We will focus on Philip. Philip would marry also marry a Portuguese princes Mary of Portugal (yes, she was his cousin we'll get into that later). She died quite quickly and had one son who wouldn't survive his father. After that the Habsburgs saw an opportunity, Henry VIII of England had died and his catholic daughter, Mary, was on the throne looking for allies with the protestant catholic split in her country. Philip married the older queen and if things had turned out differently the Habsburgs would have also been on the English throne. But sadly for Philip, Mary would die without any surviving children. Philip seems to be quite unlucky with his wives as his third wife a French princess would also die, but she did give him two daughters who he married of to the Duke of Savoy a ruler in what is now north western Italy and the other went to another Habsburg to fortify the connection with the other branch of the Habsburg dynasty that had its base of power in Austria-Hungary. His fourth wife was also a Habsburg and she would give him 4 children, one of which survived till adulthood. This was Philip III, who would also marry a Habsburg, Margeret of Austria, with whom he would have eight children.

Well why did they marry so many other Habsburgs, why all the incest? Well the Spanish Habsburgs didn't have all that many options. At this point there weren't that many catholic kingdoms, there was Spain, France, there was Habsburg Austria-Hungary and Poland. And the other Catholic rulers were all dukes or counts. Something that the Habsburgs saw as being of lower status. Even the Polish royalty wasn't seen with as much prestige becuase that was an elective kingship. So why didn't they marry French and Polish princes and princesses then? Well, France was a big rival so they were often at war so marrying them wasn't an option and Poland was rather far away. Spain in this period was often at war with France and England so an alliance with the Habsburgs of Austria would be far more helpful. If you married Poland you would also have to help them with their wars all the way over in eastern Europe. This made such a marriage less attractive. Although the Spanish Habsburgs did on occasion marry into the Polish royal family.

There were certain marriage blocs that developed, a protestant, catholic and orthodox bloc. With the Orthodox being mostly confined to Russia after the Ottoman conquests of the 15th and 16th century and the rest of Europe being more or less split into a northern protestant half and a southern catholic half. In the middle of this was Germany. What is now Germany was split into dozens of even hundreds of smaller states which has to do with the nature of the Holy Roman Empire which is a story for another time. Some of these states were catholic and some were protestants again mostly split north to south. Because there were so many of them, they would pretty much always be a German marriage candidate available and because of Germany's central location in Europe they would be possible allies for most other states.

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