Torsion manuballistae are finicky weapons. You can use them to good effect, but they hog maintenance time and require exotic supplies. There is evidence for more conventional crossbows in Roman times, and I suspect they were not widely used because they didn't find a military niche. Highly trained soldiers (like the palatine troops or royal retainers) could spend the time and effort to learn shooting the bow or using the sling. Poorly trained troops might lack the support system necessary to produce and maintain something as complex as a crossbow. And there were no urban militias in Rome.