Because it would have violated treaties with Portugal. The Treaty of Tordesillas (1494) established a line of demarcation in the Atlantic. Spain was granted rights to lands westward, Portugal to the east (it travelled through Brazil this allowing colonization there). The line confirmed Portuguese feitorias in Africa and their desire to get to the Indian Ocean and beyond. It also confirmed papal bulls to both nations that supported their expansion as long as they spread the Gospel in lands the reached. In 1529 the Treaty of Zaragoza extended that line around the poles but not perfectly (Portugal received 191 deg. and Spain 169 deg.). Spain's eventual conquest of the Philippines violated the boundary, but did not target an area of Portuguese trade.