Because he was about to give the world free energy because he figured out how to tap in to the earth energy and that would of made all free and the rockefellers could not let that happen so they stoked everything from him and said Edison invented everything when in reality it was Tesla.
Discussion
I tend to agree with this story, only as far as I know it was mostly J.P. Morgan who financially ruined Tesla - recalled all the financing he gave him previously, and made sure no one else would invest in him.