I'm guessing it's a combination of the fact that dairy breeds aren't specifically bred for temperament, the fact that the calves are all raised by hand, and that the bulls are around people on a regular basis.
Your beef bulls get put out to pasture and you aren't working around - or walking among them - on a daily basis. Our dairy bulls run with the milking herd, get locked in the holding pen, come through the barn, etc. I'm always walking next to them, giving them a slap on the rump to move over, working in close proximity to them - they're used to people and it takes away some of the natural fear. Sure, they're still not completely comfortable being nearby, but they have lost most of that natural inhibition and fear of being close to and being touched by a person.