The Treaties of Tordesillas (1494) and Zaragoza (1529), basically. The Treaty of Tordesillas divided the New World and trade with Asia/Pacfic regions between Spain and Portugal, who were at the time the naval superpowers of Europe. That’s why the Portuguese established trading ports in Goa, India, for instance, while Spain couldn’t go much further than the Philippines (then the Spanish East Indies). The Treaty of Zaragoza settled the antemeridian set by the Treaty of Tordesillas, basically giving Portugal control of all Asia, including the so-called “spice islands”. Spain got most of the Americas and the Pacific. This is also why Spanish voyages tended to go across the Atlantic to the Pacific, instead of around Africa and across the Indian Ocean.

Later on, as other countries established trading routes (Dutch, English, French, etc.), there began to be more encroachment onto these areas, as the spice trade and other trade goods from Asia (silks, porcelain, tea, among others) were very lucrative. Private companies like the Dutch and British East India Companies became very powerful as a result.

Edit: thanks for the silver!

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!