An act of god.
Radiant and other Quake engine editors (Hammer etc) all use BSP brush based geometry. EQ zones use single continuous mesh geometry (like a big inside-out 3D model made in 3DSMAX).
There's some debate as to whether the zones were created in a brush engine (which I myself am against) but what's for certain is that the finished zone geometry shows no signs of being made like that, so it'd not be possible to turn them back into editable brush geometry.