Kant was a product of his times and his views on Republics is the result. For hundreds of years in Europe, monarchies were slowly giving way to more distributed rule ala the Magna Carta, parliaments (initially temporary insitutions to resolve certain issues), etc. A few hundred years of this slow progression existed before Kant, often with decades of setback in a country before proceeding again. Distributed rule therefore likely seemed inevitable to him because no matter how long a monarchy reestablished power, it inevitably gave way again.