it is suggested that the Ottoman capture of Constantinople was instrumental in Christian countries seeking new routes as it made the capital and the Black Sea more of a tricky proposition. So if this is 1100-1200 we're still a couple of hundreds years off that event.
It's a question of product, too. Countries only got seriously into the trading game when they had something valuable to offer in return. Portugal's early contribution to the trade was slaves, captured from the west coast of sub-Saharan Africa. They were the first Europeans to really explore south of the Sahara. Pushing ever-farther south for new targets led to rounding the Cape of Good Hope, and opening a new route to India - bypassing the tariffs of the Constantinople and Alexandria routes.
