The skills required in baseball are a restriction to most females. Most will never be able to throw overhand at more than 80MPH, or throw it 300+ feet from outfield to home. The smaller field in softball along with the whirlibird pitching makes it more suitable for the naturally weaker sex.
Now, I am not in ANY way saying softball is easy - it absolutely is not, and as posted above most highly skilled baseball players have extreme difficulty hitting against highly skilled softball pitchers. Plus the short base paths and smaller field overall make it an incredibly fast game. It absolutely amazes me that the 3B in softball can play 30 feet from the plate and pick up a sharply hit ball quickly enough to make the throw to first.
My sons have been playing baseball for well over 10 years now, and I've seen some fantastic female players over the years. But as they get closer to puberty and adolescence, the athletic advantage that many of these young ladies have over their male counterparts diminishes. I was at a 12U game a few years back and commented to my wife that the best player on the field was the young lady in center field. The guy next to me said, "yeah she already threw 5 innings of no-hit ball and hit a HR and triple."