To be fair, Xsplit (closed source) was the one that was there as streaming got established, though OBS has taken over these days by being better and free (as in beer - though also being open source obviously helped get there).
XSplit emerged slightly before OBS Classic but OBS innovated with a far superior game capture implementation on Windows, supporting shared texture hooking across D3D8-D3D11 and OpenGL, which helped it gain massive traction with the Twitch crowd. After retaining a large user base it was eventually rewritten as the cross-platform OBS Studio.