Reddit reviews The Venetians: A New History: From Marco Polo to Casanova

Paul Strathern argues that, among other things, the fall of the ERE and rise of the Ottoman Empire was one of the tipping points in Venetian history. Venice, in many ways, was not ideally situated for the type of commerce it mainly engaged in, to the point where the Council of Ten considered leaving and being the Republic of Venice somewhere other than the Venetian islands.

Part of what had previously made Venice successful were lucrative trade deals that the Republic had made with the ERE, including trading rights in Constantinople and other major ports. With the ERE on its deathbed and the Ottomans rising, the Venetians had hoped to renegotiate these trade rights with the Sultan, which is likely part of the reason why they delayed a relief fleet which was meant to go to Constantinople. However, that did not work out as well as they had hoped and the Ottomans were far less willing to negotiate trade deals than the Greeks had been.

In addition, the 15th century was filled with many economic and technological changes, one of the most important being the Portuguese navigating a route around the Cape of Africa. So diminished trade opportunities, foreign competition, and a less-than ideal geographic location were factors to the decline.