The decline of the Portuguese was brought about by several factors. The local advantages gained by the Portuguese in India were reduced with the emergence of powerful dynasties in Egypt, Persia and North India and the rise of the turbulent Marathas as their immediate neighbours. (The Marathas captured Salsette and Bassein in 1739 from the Portuguese.) The religious policies of the Portuguese, such as the activities of the Jesuits, gave rise to political fears. Their antagonism for the Muslims apart, the Portuguese policy of conversion to Christianity made Hindus also resentful. Their dishonest trade practices also evoked a strong reaction. The Portuguese earned notoriety as sea pirates. Their arrogance and violence brought them the animosity of the rulers of small states and the imperial Mughals as well. The discovery of Brazil diverted colonising activities of Portugal to the West. The union of the two kingdoms of Spain and Portugal in 1580-81, dragging the smaller kingdom into Spain’s wars with England and Holland, badly affected Portuguese monopoly of trade in India. The earlier monopoly of knowledge of the sea route to India held by the Portuguese could not remain a secret forever; soon enough the Dutch and the English, who were learning the skills of ocean navigation, also learnt of it.

Source: NCERT