Preference, cost, and ease of installation. Imo screws are easier to install, and are cheaper in smaller sizes. Most people here use 1/2" acme screws, which are cheap. As lengths get up around 5 ft, screw whip becomes an issue. At this point most seem to use rack and pinion, rather than go to larger screws, which cost more, and require more power to accelerate due to the greater inertia.
If you compare rack and pinion to ballscrews, rack and pinion is much cheaper. However, with rack and pinion, you typically need a gear reduction, which adds cost and complexity.
I'm currently working on a design with a 6 ft X axis. I originally planned on using 3/4" acme. Then started to look at rack and pinion as an alternative. But the complexity of the gear reduction turned me back to the acme, which in my case will be slightly cheaper than the rack and pinion would have been.