Crimea is part of Russia now because
(1) It has been part of Russia, with a short break from 1991 until 2014. In other words Crimea became part of Russia before the USA existed.
(2) The citizens of Crimea voted, not just by a majority, but by an overwhelming majority, to become part of Russia again. (Incidentally, no one ever asked them in 1990-1 whether they wanted to leave Russia and become part of Ukraine).
(3) The great majority of Crimean people speak Russian, and consider themselves Russian.
(4) Over the centuries, huge quantities of Russian blood has been spilled to make and keep Crimea part of Russia. The number of Russian soldiers who died to conquer Crimea from the Tatars, to defend it from the British and French, and to defend it and then recapture it from the Nazis, is at least comparable with the total US casualties in both world wars.
If you don't believe that the plebiscite was run fairly, that is your problem. It was monitored by international observers who did not raise any objections.