TLDR; What lead to the Annexation was a century of increasing American economic and political influence on the islands. The islands where mostly interesting to the US due to the sugar trade and their strategic benefit in pacific warfare. The latter lead to their annexation during the Spanish-American war. Prior to that a coup mostly of American plantation owners have overthrown the native monarchy in order to be annexed into the US. The annexation was only one event in Americas transformation from isolationism to a global power and just as the others it was accompanied by heated ideological debates./TLDR

Hawaii was unified by a king in the early 19th century. During his reign American influence over the islands grew rapidly. Beginning in 1820 American missionaries entered the islands. Since 1842 the US guaranteed the kingdoms independence.

The US used this influence for favourable trade deals: In 1875 all tarrifs on sugar trade from Hawaii to the US were removed. This made the economy of the king extremely dependant on America, but it was profitable for white plantation owners that began moving to the islands.

In 1887 the US went as far as recieving Pearl Harbour as a naval base. The strategic importance of Hawaii for pacific dominance lead to American expansionists demanding it's annexation. The concession of pearl harbour was connected to the enactment of the so called "Bayonet Constitution" which was enforced by white militias. This document took most of the kings powers and gave it to the mostly white, American government officials.

Now in 1891 a new queen inherited the throne. She tried to reinforce the monarchs power but failed due to the resistance of the American influence. Her attempt on reintroducing tarrifs failed terribly and threw the countries economy into a crisis.

That lead to a coup d'état which overthrew the queen and a "republic" lead by American plantation owners with the purpose of being annexed into the US was founded. Cleveland opposed the coup, his administration ruled it to be illegal, but an congress investigation ruled only the queen to be quiltya and the officials of the republics refused to back down.

Now in the 1890s there was a rise of imperialistic ideologies in the American public. That was preceded by an extreme rise of colonial influence of European powers in Africa and Asia. 1897 William McKinley was elected president. He surrounded himself with American imperialists and still believed in American Manifest Destiny, the expansion of the US into the west, to the pacific. (Christof Mauch: "Die amerikanischen Präsidenten")

In 1898 McKinley intervened in a Cuban rebellion against Spain, on Cubas side. His exact motives for doing so are up for debate, but it is clear that he waited until a majority of Americans approved of it ^1. The casus belli (reason) for this was an explosion of an US ship in Cuba which was said to be done by the Spanish. This marked a turn in American politics away from isolationism and introduced the US as a global power (later confirmed e.g. by the U.S. intervention against the Boxer rebellion in China and most notably in WW1.)

The colonial posessions were also legitimized by the idea of White Man's Burden. This was opposed by a stong anti-imperialist lobby, the AAIL which was mainly founded to oppose an annexation of the Phillipines.

Now McKinley wanted to annex Hawaii, but a senate vote in 1898 failed as over half of Hawaii's native population signed a petition to not be annexed and as such he didn't get a 2/3rd majority.. In order to still achieve his goal he used a [Joint Resolution, a majority of votes in Congress and Senate] (http://www.sourcebooks.fordham.edu/halsall/mod/1898hawaii1.asp) to approve the annexation.