Why does so much of European nobility seem to have German origins?

Current European Nobility is perhaps the only social group with a maniacal obsession with maintaining (and tracking) their ancestry spanning, sometimes, over a thousand years. Consequentially, many English families can trace ancestors back to the Norman Conquest, and many Italians back to Barbarossa's invasions. Given that European Nobility had a vested interest in affirming the validity of their pedigree, a lot of genealogies jump through some fanciful hoops to connect to Charlemagne.

You're probably thinking of the British Royal Family, which ascended the throne thanks to the seemingly insignificant marriage of their ancestor, Frederick V Elector Palatine (and briefly, King of Bohemia) and Elizabeth, second child of James the VI (of Scotland) and I (of England). However, they weren't the only family to go through elaborate generational gymnastics.

Of all nationalities, I'd say Italians were the most over-represented in European Nobility. Noble (and Royal) houses in search of rich dowries had a habit of marrying Italian princesses, whose titles were very often far less impressive than those of their spouses. Multiple Kings of France would have Italian descent, as Henry II married Catherine de Medici, and Louis XII who came to the throne after the death of his cousin Charles VIII had an Italian Grandmother (Valentina Visconti, who had married his grandfather Louis, who was the younger brother of King Charles VI). The House of Este in also related to almost every other Royal house in Europe, but they themselves are a Junior Branch of the house of Welf, following the 11th-century union of the Elder House of Welf, who were themselves a junior branch of Burgundian Royalty, with the House of Obertenghi, Lombard Nobility who claimed descent from the Roman Gens Attia. The junior Welf (German) branch were first Counts, and eventually rulers in Bavaria who contested the claim to the Holy Roman Empire at one point, while the Este were made Magraves of Lombardy during the Ottonian dynasty. The Este are related by marriage to the Hapsburgs, and their members married into the the house of Hannover (consequentially, the English royal family is descended from them) and, more recently, the Duke of Bavaria and the King of Belgium.