There are multiple problems. The biggest problem in my mind is that outcomes are not tracked well. That means that there's no medical feedback loop to measure the long-term effectiveness of... anything. And no good way to do retrospective studies to prove new measurement techniques. And also the laws for patient protection makes it incredibly difficult to get even anonymized data for researchers to use.
The next biggest problem is that there's a gold standard of treatment, and it's hard to get enough research to move the gold standard. In this case, the gold standard is essentially a diameter measurement, based on picking the visibly largest slice of a data set which happens to have an axis determined by the patient's somewhat random orientation in the CT scanner. Axial, Coronal, Sagittal. Those are not perfect, anatomical orientations - they're rough. Most radiologists won't take the time to create Multi-Planar Reformats along oblique or double-oblique angles. And even if they did, they'd still be eye-balling the measurement.
It's not very hard from a regulatory standpoint to create tools to allow a radiologist to click here, click there, and show a measurement of distance. Then the radiologist can look at a similar measurement from a different radiologist on the previous scan, and say whether it's bigger or smaller now. (But some PACS still don't handle anisotropic MPR correctly, which just blows my mind.)
It's vastly harder from a regulatory standpoint to say in a medical product for commercial use, "Hey, we automatically measured the volume of this tumor by automatically finding the boundary of the tissue and automatically ignoring the connected blood vessels and neighboring bone, and then we automatically found the largest cross-sectional diameter, and then we automatically compared the same measurements against the automatically non-linearly-registered longitudinal previous scan... diagnosis: the tumor is not growing, just changing shape slightly."
People think their CT scans are examined for hours by a team of doctors, like on "House." In reality, you're lucky if your CT scan gets 5 minutes of investigation, and that's by a radiologist who was trained in med school to read X-Rays, not CT. And your oncology head CT may be read by a radiologist who specializes in abdominal pediatric Ultrasound. And there's almost certainly no double-reading for Quality Assurance. And there's no rating on radiologists to tell you who has the most accurate diagnosis. And he's probably using 2D measuring tools on 3D data - because that's what they're familiar with, and that's what the reconstruction software manufacturers provide, and that's the medical gold standard. On a shared computer, where some other radiologist (or resident!) might have changed the display settings in a way he didn't notice. And he's probably been reading images for 14 hours straight.