Hi,
Interesting thought. Something like a roman scorpio would be less expensive to produce, material more readily available, and easier to move than artillery. These could all be positives.
Maybe the choice to focus on heavy artillery was accuracy. While not totally precise, it could often be aimed and distance controlled by powder charge and angle. And the balls can bounce down the field once they impact the ground, offering additional potential for damage.
However, the scorpio idea gets me thinking about archers too, and what a regiment of archers could do by raining arrows down, or flaming arrows. I’m sure others could share reasons why that might be impractical, though it’s interesting to think about.