OpenGL 2.0 dates back to 2004, it's almost 17 years ago. Since then, the graphics landscape has changed dramatically.
Nowadays, we take some modern OpenGL features for granted, for example framebuffer objects. Without them, kwin won't work as desired, for example software output rotation will be broken, etc.
This changes make it official that kwin requires OpenGL 3.3 on desktop.
OpenGL 3.3 was chosen because it includes some interesting features that can be useful in the future, for example timer queries, and it's arguably old enough (2010) so not many people should be affected by this bump.
With GLES, things are complex. GLES2 is still somewhat popular.