It certainly was not for lack of trying. Prior to the outbreak of Spanish-American war, McKinley had proposed to outright buy Cuba from Spain. This is not to say McKinley wanted war, rather McKinley saw Cuba as strategically important and therefore wanted a Cuba that is at least subservient to the United States. But the USS Maine incident forced his hand due to public anger, and his proposal to purchase Cuba was no longer viable. McKinley then demanded Spanish withdrawal.

But what to become of Cuba? This is where the US Congress couldn't immediately agree. At one point, they they were ready to recognize the rebels, which of course brought the possibility of an independent nation that wasn't friendly to the US. Yet Congress didn't want the US to appear imperialistic.

The compromise became the Teller amendment, disclaiming US intent to annex the island unless the US was forced to intervene militarily. Even Teddy Roosevelt, who was eager for war, supported the amendment lest "it seem that we aremerely engaged in a land-grabbing war.''

The peace negotiations in Paris at the end of the Spanish-American war was made much more complicated by this Teller Amendment. It made direct annexation of Cuba impossible, yet the US was able to obtain Puerto Rico, Guam, the Philippines. Spain was loathe to be burdened with having to deal with the aftermath of the war in Cuba, especially not if it had to be burdened with the debts of Cuba -- whereas the US took over the fiscal responsibilities of obtaining Guam and the Philippines.

So it was a mix of yellow journalism, Congressional indecision, and McKinley being forced into war.