There are two issues with round bales that come into play around here. One, they don't transport easily, because the dimensions are awkward for a semi-trailer.
Second, especially around here, the humidity is only high enough for a few hours a day so that the hay doesn't crumble to dust going through the baler. A round baler, at least the ones I've been around, need to stop and tie off the strings once the bale is completed. Then you eject the bale, and start a new one.
The square balers are more expensive, but you can keep running when a bale is completed and being tied off. When fighting time, that makes a substantial difference, at least 50% more throughput. It can be the difference between green hay and brown hay, as well. Combine that with the easy of transport, the square bales win.
