The United States has a long history of viewing binding international treaties with skepticism, and Congress has often rejected international accords that its members feel might restrict or in some way inhibit the growth of U.S. industry or foreign policy. This reticence to submit to being governed by international accords stems partly from the notion of American Exceptionalism, the idea that the United States is both morally and practically superior to other nations and therefore should only submit to its own laws, or risk losing its position as a pre-eminent economic and military power. This impulse also stems from the United States’ historical isolationism, which stems from the earliest days of the republic, and the notion that the so-called "American experiment" was in part a creation of a new world order, and even a new world.
This isolationist and exceptionalist mindset was famously demonstrated by Congressional Republicans in the early 1920s, when they refused to sign on as members states of the League of Nations, which President Woodrow Wilson had created to keep the peace after World War One. Later, after World War Two, the United States did sign on as a member state of the U.N., and to NATO, partly because the U.S. has exceptional power over the decisions and actions of both of those international bodies. Even so, the United States has still not signed on as a full member of the International Criminal Court, which the U.S. helped create, and which puts the United States in the odd position of being in same group of holdouts that include famously authoritarian human rights abusers, like North Korea, Sudan, Iran and Saudi Arabia. The reasons the U.S. gives for not signing onto the International Criminal Court are very similar to those it gave for refusing to sign the Kyoto Protocol: namely, the United States maintains that it cannot make its own government and laws subservient to international law, because doing so would undercut its sovereignty.
With regard to the Kyoto Protocol, the United States was particularly displeased with the specifics of the agreement, which put the onus for decreasing green house gas emissions on rich, industrialized nations like the United States, without forcing developing countries like China and India to make similar concessions. So you might ask what has changed between then and now, when the United States has signed onto a tougher and more comprehensive climate change bill with the Paris Climate Accord?
The short answer is that we now have a President (Obama) who has made combating climate change a central pillar of his domestic and foreign policy, because he understands the dangers of doing nothing. Moreover, the science behind climate change and the very visible evidence of the havoc it is creating in our world is much more obvious now than it was back in the late 1990s, when the Kyoto Protocol passed.
Aside from the fact that the existence of man-made Climate Change is now pretty much undisputed among mainstream scientists and government bodies, the fact that the United States was able to convince China, the second largest green house gas emitter behind the United States, to sign onto the Paris Accords and agree to equal reductions in emissions, made the United States and other industrialized nations more comfortable signing the agreement.
The thinking was that if everyone agrees to meet certain benchmarks, then no country would be disproportionally disadvantaged by agreeing to reductions. Another reason that the U.S. agreed to sign the Paris Accord stems from the fact that whereas alternative energies were not seen as profitable back in the late-90s, now they are a huge industry, and countries that adopt and create these technologies see them as a major driver of economic growth, instead of as a drag. In order to make these industries profitable and sustainable, however, individual countries have to make polluting costly, to incentivize their own domestic production of alternative energies, electric cars, and other now-mainstream products and services that benefit from an international limit on carbon emissions.
Back in 1996, many politicians in the United States saw no economic benefit from signing the Kyoto Protocol. They worried that it would negatively affect American industry. With the Paris Climate Treaty, America was the main driver of negotiations, and it was a priority of an American President. Just as importantly, the American public now generally supports efforts to combat climate change, and to profit from technologies that help to do so, whereas the public was more skeptical back in the 1990s.