Why didn't King Ferdinand take over Portugal during the Spanish Reconquista?

Complementing the answers above:

The different kingdoms were a personal union, not an unified country. For example, the Invincible Armada was financed by Castile, as the cortes of Aragon did not support King Philip II's war. So, waging an offensive war required raising support separately on his various domains (expensive and time-consuming), OR, having only some domains participating (and then it could be, e.g., only Castile versus Portugal, not as overwhelming).

Besides, I think Queen Isabella still had power over Castile, it would not be only his decision.

Portugal was historically allied with England — this could make an alliance of Portugal and France less likely. Why would Spain deliberately risk moving Portugal and England to the French side? Check how many times Spain and France warred, including in Italy — even if these wars start a little after your time period, the conflict of interests was already there.

In 1494, the Tordesillas treaty regulated the colonial competition between Spain and Portugal. The Treaty implied they respected each other's naval power — peacefully dividing the world would be more profitable and safer than just 'the first to land grabs he land' or grabbing land by force. The Portuguese were navigating before the Spanish, and already had a profitable African trade, and the Cape Verde, Madeira and Azores islands, and had already beaten the Castilians at sea a few decades before. A war with Portugal could imply a significant naval effort on the Atlantic, to defend the Canary Islands or to allow troop supply/movement by ship. In short, it would not be easy on the naval side, it could harm both countries trade/colonial efforts, and again, it would divert Spanish resources from more urgent issues in the Mediterranean.

The question of legitimacy is not to be forgotten too. Not everybody was 100% Machiavellian: many may have truly believed that Portugal was not their business — if not by a sense of justice, then because they already had trouble enough running their different kingdoms, or because the complex issues between Isabella and Portugal already had cost much to be settled — Isabella could have lost her throne to a Portuguese supported claimant, or she could have unified Castile and Portugal via marriage, instead of picking Ferdinand/Aragon. When they inherited the Portuguese throne in 1580, they had another situation: not pressing their claim could show weakness and encourage independence thoughts on their many newer domains.

He (they) tried somehow, but it didn't work due of a handful of unfortunate deaths.

Ferdinand and Isabella didn't try to invade Portugal - and other answers explain quite well why it would have been a bad idea - but they arranged the marriages that could have lead to the unification of Castille, Aragon and Portugal by marriage.

Isabella of Aragon, the older daughter of Ferdinand and Isabella, married the king of Portugal. Hadn't both she and her only son Miguel da Paz predeceased Isabel and Ferdinand, he would have inherited Castille, Aragon and Portugal.

In fact, the unification of Castille, Aragon and Austria was a rather unexpected outcome of the matrimonial policy of Ferdinand and Isabella. The unification of the Iberian Peninsula under a Portuguese king would have been more likely - and an interesting twist in Spanish history.

Takeovers of this kind often involve "trigger events." There was "not quite" a trigger event for the Spanish takeover of Portugal during Ferdinand of Aragon's time, and there was one late in the 16th century, when Portugal was taken over.

In 1580, the direct line to the throne of Portugal died out (the trigger event). There were a number of "cousins" to the royal family, one of which was Philip II of Spain. It was in this "role" that he took over Portugal, not on behalf of Spain, but under a "personal union," whereby he was king of Spain and Portugal.

In Ferdinand of Aragon's time, on the other hand, the Portuguese royal family was intact. Also, Ferdinand, as king of Aragon, was actually more interested in events to his east, that is, regarding the balance of power in Italy, where both France and Spain had interests. Portugal was a poorer, less populous country by comparison, meaning that it was not as high on the list of Ferdinand's priorities. His interest in Grenada was partly because of his wife Isabella, partly because unlike Portugal, it was occupied by Moors (and Jews), rather than fellow Catholics.

Leon and Castile had been united much earlier in 1301, not in Ferdinand's time, although his marriage to Isabella united Aragon with these two territories. And he did have a part in bringing the Spanish-speaking (but not French-speaking) part of Navarre into Spain.

The "not quite" in the first paragraph refers to the fact that a merger of Spain and Portugal might have happened if things had worked out differently. First, the original heir to the Spanish throne, Juan of Asturias, died early, making his sister Isabella of Aragon the heiress. Second, Ferdinand and Isabella initially offered King Manuel of Portugal the hand of Isabella's younger sister Maria in marriage (whom Manuel later married), but Manuel insisted on Isabella. Third, Isabella consented to marry Manuel if he would agree to expel the Jews for Portugal (he did). This prospect could have brought about the union of Portugal and Spain, except that Isabella and her son (the heir to the Portuguese throne) both died young. The middle sister, Juana, then inherited the Spanish throne ahead of Queen Maria of Portugal, and through Juana, Charles V

Put another way, it was a "near miss," based on "luck of the draw." But since Manuel of Portugal had agreed to align his policy towards Jews with that of Spain, Ferdinand and Isabella weren't about to try to take Portugal by force.

The great reason is that after Granada fell, France was the great danger. Spain needed alliances to defy the big European kingdom. France´s king wanted to conquer Italy. The Kingdom of Naples was sinking without an heir and the situation was desperate. Naples was considered also a strategic and important kingdom to face the Ottoman danger.

The Portuguese and the Austrian Hapsburgs were considered potential allies to face France's growing power and the menace of the Ottoman empire. The matrimonial alliances policy were the idea of the dangerous moment.

To the Kingdom of Naples was sent the new Spanish young talented commander Gonzalo Fernandez de Cordoba. He is the most important leader in Spanish military history, the father of the famous Spanish Tercios. Key in the Conquest of Granada and key in the defense of Italy.

Many people consider the conquest of Granada as the Victory of the Catholic Monarchs. Even the film Conquest of Paradise was showing that, but they forgot about this last character.