There are at least three main reasons why the Portuguese attempts to trade with India were ultimately successful. The Portuguese were strongly motivated to seek this trade. They possessed strong skills and technology. Finally, they were willing to be ruthless in their attempts to dominate trade in the area. Let us look at each of these factors in turn.
First, the Portuguese were very motivated, perhaps in part by their very marginal status in Europe. Portugal was confined to a western strip of the Iberian Peninsula, which is itself cut off from Europe in a sense by its geography. Portugal had no clear way to achieve wealth and power within the context of the European political and economic scene. Therefore, Portugal was strongly motivated to try to trade with India as a way to improve its position in Europe.
Second, the Portuguese had the skills and technology they needed to succeed in the Indian Ocean. They studied navigation and seamanship, most famously at Prince Henry the Navigator’s school in Sagres. Their ships were more suited to fighting than the ships that were present in the Indian Ocean when they arrived. Their cannon were better than the weapons that their competitors possessed. All of these factors gave them technological advantages.
Finally, the Portuguese were not constrained by ideas of fair play. They felt that they had the right to dominate anyone they encountered in India. They felt this, in large part, because the people with whom they were dealing were not Christians. Therefore, they were willing to act in rather brutal and ruthless ways in pursuit of a monopoly of trade with India. They won this monopoly by what we would think of as piracy as much as they won it through trading acumen.
So, we can say that Portugal was successful in its attempts to trade in India because it had better skills and technology and because its motives and ideologies allowed it to pursue that trade in very ruthless ways.