The core OpenGL function calls are the same between Windows and Linux. So what is the difficulty to support it in Wine ? Well, there are two different problems :
the interface to the windowing system is different for each OS. It's called 'GLX' for Linux (well, for X Window) and 'wgl' for Windows. Thus, one need first to emulate one (wgl) with the other (GLX).
the calling convention between Windows (the 'Pascal' convention or 'stdcall') is different from the one used on Linux (the 'C' convention or 'cdecl'). This means that each call to an OpenGL function must be 'translated' and cannot be used directly by the Windows program.
Add to this some brain-dead programs (using GL calls without setting-up a context or deleting three time the same context) and you have still some work to do