http://www.sgi.com/tech/openGL/paper.design/subsection3_4_1.html (Silicon Surf Promotional CD, 01/1995)
OpenGL is based on Silicon Graphics' IRIS GL. While it would have been possible to have designed a completely new API, experience with IRIS GL provided insight into what programmers want and don't want in a 3D graphics API. Further, making OpenGL similar to IRIS GL where possible makes OpenGL much more likely to be accepted; there are many successful IRIS GL applications, and programmers of IRIS GL will have an easy time switching to OpenGL.