The American educator and humanist John Dewey offered us a quote brimming with characteristic 20th century American optimism: "Every great advance in science has issued from a new audacity of the imagination." I would be unsurprised to learn if an early Venetian humanist (or political scientist) hadn't offered us an analogous quote along the lines of, "Every great advance in Venetian government has issued from a new political crisis."

There are two macronecessities in Venetian political history: effective government and representative government. The desire for effective government is what drove the venetians to add increasingly specialized appendages to government, while the desire for representation is what drove the venetians to ensure that as many different parties as possible were privy to the actions and decisions of those specialized appendages. In other words, this is not a time when government was professional, and therefore the venetians preferred large committees where everyone could bring different skills and perspectives to the business of government; nor was it a time when enfranchised people had any sort of faith in democratic process, so electing or choosing a person and sending them off to do a job was not preferred to large committees where everyone could instead continuously monitor each other.

Was this effective? Not particularly. But there is a sequence of events punctuated by conflict, crisis, and the need to resolve these crises in a way satisfactory to all interested parties which led Venetians to structure their government this way. We'll get into that below.

But first, I want to lay down a few points of order: are we entirely sure no other medieval government changed just as the venetian government did? We cannot discount the possibility that the Venetians were simply much better at writing things down (we can, if we'd like, rationalize that the pervasively mercantile vocations of the citizenry seems to have imparted an intergenerational obsession for meticulous record keeping). From the earliest medieval period, from the Kingdom of the Lombards to the Carolingian Empire, we know a lot less than we'd like on how government did or didn't function on the Italian mainland (with information on the rest of Europe even worse for the wear). Sure, we can draw an inference here, examine a document there, read surviving chronicles (almost always writing from secondhand sources after the fact) but we do not really know for certain what sort of administrative, organizational, or political changes occurred which made one monarch more successful than another. This means, just as an example, that we do not know with precision what sort of improvements or reforms the Lombard rulers Desiderious and Adelchi undertook to so successfully neutralize the Eastern Empire's strongholds on the Italian peninsula, nor do we know with certainty how much of the Carolingian government system was preserved in Italy after the Frankish empire definitively collapsed (irrespective of the fact that it was definitely a major watershed systemic change). More often than not, we not have the sort of meticulous detail of blood feuds, riots, and political changes which instead venetian writers like John the Deacon describe to us.

Where we do have documentation (it picks up in and around the tenth century) we actually know that there were moments when political systems and institutions did change significantly all over Europe: A prominent example is the incredibly efficient Ottonian empire in Central Europe-Italian axis (itself catalyst of enormously far-reaching consequences, and less efficient successors) but also events like the Norman Conquest of England, or even the changes intrinsic to the nascent Reconquista process in Ibieria.

So the point is that we generally shouldn't be surprised that institutions change over time. The question is then what, specifically, drove the Venetians to change and modify their system of government, and what drove them to change and modify it in the precise way they did?

I've discussed similar questions like this one on the mechanism of the Doge's election, this more generic answer on the broader venetian political system, and this similar answer on how the Venetian govornment functioned if you (understandably) don't feel like getting into the wall of text I've written below. It's posted as a reply as I'm past the wordcount.