Also, powers like Venice, which sought to sew up the trade in Eastern goods, were a major reason for relatively peripheral nations like Portugal and Spain to seek other channels to access those markets.
The Venetians didn't perceive a need to go roaming around looking for routes around Africa, across the Ocean Sea, or picking their way through ice packs looking for a Northwest Passage. They already owned the direct route!
Eventually of course the hard way the Iberians and other Atlantic nations had to take paid off. But who foresaw that in 1500?