The CT scanner was initially designed to scan human bodies which have by essence a volume (3D)... like sculptures. 

As paintings are essentially 2D-objects, CT scanning them with a medical CT scanner is pointless as they are too thin to allow the extraction of any interesting features. 

Moreover they are often too large to enter the donut-shape opening of the CT scanner. 

So, to study paintings, the use of century-old conventional x-rays remains appropriate but not the CT scanner.