I'm not sure I ever really noticed, but I think I can see why that might be the case. Games are typically outdoors, and in the sun, and the weather starts getting warmer. Which I think encourages some girls to want to wear cute outfits with their team logos on them. The clothing also seems pretty adaptable, more so than other sports. So not just jerseys, but t shirts. Like those shirts with the different colored sleeves. Reminds me of girls playing softball in the 70's. That shit is hot.
The atmosphere is also generally pretty relaxed. You're out in the open, there's the food, and baseball just seems more conducive to girls having a good time. But you look at a basketball game for example, few of the fans are wearing basketball jerseys, or the team's colors. Sports like basketball and hockey just feel more like "keep your jacket on" type games. Women dress down, but don't really look like they're having that much fun.