Distance is certainly a factor - a 65 mph softball from "our" distance and a 95 mph baseball from "their" distance provides about the same (very small) reaction time from a hitter. But, perhaps more importantly, a pitcher can do so many more things with a softball than a baseball.
Ursusminor was (and is) primarily a third baseman by trade, a good first baseman, but, she did a little pitching too. "Bear" in mind, she was never accomplished enough to pitch in a high school game, but, middle school, some summer ball, yeah - plus, she would do batting practice in high school if needed. But, even at her fairly remedial level, she could throw fastballs at different speeds, changes, drops (I HATE catching drop pitches during winter indoor practice), curve/screwball, and a rise. Yeah, a rise - softball pitchers can make the ball go up.
I'm not sure of all the reasons you can do so many things with a softball - I think the big seams have something to do with it, along with the fact that the ball is so big, that there's more surface area for you to move your hand into slightly different positions. Releasing the ball near the bottom of an underhand arc may help too. But, for whatever reason, a super pitcher like Joyce can make the ball sit up and beg, fetch your slippers, and do your taxes.
The other factor is, baseball is based, largely, on a descending pitch, released well above the strike zone - so, a hitter generally has a bit of an "uppercut" aspect to the swing, so that the force of the swing is in 180 degree opposition to the force of the pitch. In softball, the pitcher releases the ball down by her knee, at the bottom of the strike zone, and often with a slight ascending aspect. An optimal swing will be much flatter than a baseball swing, sometimes even descending. This would be a very, very tough adjustment for a baseball hitter who has refined their swing so precisely - like a Williams or an Aaron.