While it's true that translating most CTL IDTs to DCTL is straightforward, and could be automated, that really only applies to the common ones for log formats, which are just a transfer function and 3x3 matrix. IDTs don't have to be that simple. Remember the old Canon IDT, with its 17x3 matrix? And IDTs for colour baked formats could potentially be far more complex.
If they offered direct CTL support, but didn't support everything, that could be problematic. DCTL is designed to be directly compiled as a shader. CTL is not.
OCIO is substantially built around LUTs, and can run up against the limitations inherent in that. I can completely understand why they went for a modern shader based approach. That is the direction everybody is heading, and rightly so, in my opinion.