In The Wealth and Poverty of Nations, David Landes attributes the Portuguese' floundering, at least in part, to the persecution of non-Catholics, especially Jews, and general hostility to advances in science. Coupled with the strain on resources (both human and material) that a global empire brought, this failure to maintain an edge in terms of navigation and military science meant that those nations with greater resources could out compete the Portuguese. Note that the Spanish were unable to hold on to many of their colonies in the early 1800s, while the British and French were able to hold on to theirs. The Spanish suffered from the same hostility to innovating forces that the Portuguese did. In fact, the period of Iberian Union brought the Spanish Inquisition to Portugal, which ramped up such hostility.

The Dutch, meanwhile, had no such trouble with hostility to modernizing influences, but did suffer from a similar lack of resources necessary to maintain colonies as vast as those of the British and French. Essentially, when the British decided to take Dutch colonies such as New Amsterdam and the Cape Colony, there was little chance for the Dutch to resist militarily.

TL; DR: The British and French were able to project global power to an extent that the Portuguese, Dutch, and, eventually, Spanish, couldn't match. For the Dutch, the issue was resources. For the Spanish, the issue was lack of innovation. For the Portuguese, both were issues.

not arguing about the dutch but why do you think the portuguese didn't left an impact comparable to the spanish?

Don't forget about the great Lisbon earthquake in 1755, which had enormous impact on the Kingdom of Portugal and it's colonial interests. The Last Day by Shrady is a good place to start reading about the event.