Your answer can be found in Benjamin Arnold’s book, “Princes and Territories of Medieval Germany.” (1991) - Cambridge University Press. The entire book is dedicated exactly to this question!

To summarize his main points, the Holy Roman Empire was vast and unwieldy, at times consisting of the modern countries of Germany, Italy, the Czech Republic, the Netherlands, and Luxembourg. As such, the Holy Roman Emperor could not operate from a fixed capital but had to rely upon the royal iter, or perambulation, and the entire chancery would follow with the Kaiser, drawing up and issuing royal charters such as landfrieden, or “pax Jurata.” This was a compelled land peace which meant that all of the vassals had to stop fighting and put down their arms. But often the landfrieden was ignored, and the emperor had to rely upon the princes in order to enforce the royal charters in the distant realms of the empire. As such, even the most powerful emperors understood that they needed the princes in order to maintain order and overall control. According to Arnold, “the crown had always envisaged the princes as colleagues rather than rivals in the admittedly daunting prospect of governing the huge and unwieldy German kingdom. The increasing jurisdictional authority of the princes was regarded as traditional, legitimate, and essentially to the crown’s own benefit.”

Remember, however, that in 962, Otto the Great, Duke of Saxony established the Holy Roman Empire as we know it. Thus, the Roman Empire was reborn in the Saxon model of elective kingship. According to Bishop Otto of Freising, “The apex of the law of the Roman Empire is to create kings through election by the princes, not through descent by blood relation.” Many emperors tried to make the imperial title a hereditary office, but the princes valued the balance of power between the families and wanted no one family to gain to much power. For example, you see on 1291, after the death of King Rudolf, his son Albrecht was passed up in the election of that year.

This is because the Hohenstaufen Emperors pushed them boundaries on what was acceptable, thereby angering the princes. For instance, Friedrich Barbarossa declared that imperial law overruled traditional German law, and stripped Duke Henry the Lion of Saxony and Bavaria of his titles in 1180, exiling him from the empire. Duke Henry had actually been a loyal vassal for nearly two decades but had refused to send additional soldiers to support Friedrich’s campaign in Italy.

One after the other, Barbarossa’s successors died on campaign in Italy. During the time, the Pope excommunicated many of the emperors in the war of investitures, a time of anti-popes and anti-Kings. These wars called into question the Emperor’s legitimacy, culminating in the chaos of the thirteenth century in which there would not be a crowned Holy Roman Emperor between 1250 and 1308.

Contrast this to medieval France under King Philippe IV ‘the Fair,’ otherwise known as ‘the Iron King,’ who had successfully consolidated his power over the nobles. Unlike in France or Norman England, which had hereditary rather than elective kingship, the princes of Germany thought of the Emperor as first among equals, thereby giving him enormous prestige but minimal authority.

In summary, the combination of elective kingship and imperial reliance on the princes to control the HRE, meant that the Kaisers were doomed from the start and could never have centralized their authority over such a large realm.