Because of deeply ingrained gender stereotypes that date back to the founding of the sport of baseball.

To start, softball chiefly differs from baseball in that the ball is pitched underhand and the game is played on a smaller field with larger ball that's not appreciably "softer" on the surface. Its core is softer, though, making the ball bounce off a bat differently than a baseball. While it's true that softball is a women's sport today in American colleges, softball is played by everyone, in particular amateurs, women and older men. While it can be viewed as an "easier" version of the game, it's more accurate to just describe it as different. Throwing underhand does makes it easier to throw the ball slowly. But the ball doesn't have to be thrown slowly, hence the game of fastpitch softball played in college today. An underhand delivery also allows for a wider array of pitch types than baseball. The larger ball is easier to hit but also harder to grasp and throw.

Baseball

That aside, we can dig into how the sports acquired their gender roles by going back to the start. Baseball was initially a gender-neutral, stick-and-ball child's game that slowly became gendered in the late 19th century. I'm going to lean heavily on Debra A. Shattuck's excellent Bloomer Girls: Women Baseball Pioneers for this part, because her book resoundingly answers the core of this question. Shattuck finds no evidence that baseball in its earliest days was limited to or designed for men any more so than other recreational activities like swimming or reading. But gender stereotypes and institutionalized sexism put female baseball players on an uneven footing from the start.

Baseball dates its history back to the first half of the 19th century, and college baseball dates to the 1850s. While both men's and women's colleges had baseball teams, at that time women simply had far fewer opportunities to attend college. After the Civil War, and through the 1870s, baseball began to become more organized and its rules formalized, and male stereotypes formed alongside. Hundreds of women's teams including civic and pickup teams formed during this time. Professional women's teams formed in great number as well, as managers paid both women and men to play in front of spectators. Women's teams traveled the country and drew fans sometimes in greater numbers than men.

Around the 1870s a small subset of men's teams and their sponsors began to join and form professional leagues. The most prominent of these leagues were important in formalizing the rules of the professional game, and also in institutionalizing rules around who could play, namely, white men. Initially there was not universal acceptance of the big professional leagues. Fans of the amateur sport saw pro baseball as a corruption of the sport. Fans also were used to a variety of styles of baseball. Women's teams tended to be populated by theater performers, burlesque performers, and other types of entertainers. Their teams (and many men's teams) performed a theatrical version of the game that was equally popular but was slowly eclipsed by the professional version of the men's game that prioritized elite athleticism. The best funded mens' professional teams and their leagues grew in influence and would eventually set the standards for today's professional sport.

Funding and attention from the media fed this change, as did social factors. Women's success in any field could receive backlash from reactionaries who opposed equal rights for women, and baseball was no exception. Some influential people made an explicit connection, claiming that keeping baseball a men's game would curtail women's ambitions. This pushback had a predictable negative effect on women's participation in baseball, pro, collegiate, pickup and otherwise. Even when people accepted female participation, a common theme was to portray female athletes as a novelty or funny. Shattuck writes:

James M. Bailey's comical tale about six inept female baseball players in Danbury, Connecticut, proved so popular in 1878 that Mark Twain incorporated it into his Library of Humor a decade later. Twain's reprint included an illustration showing grown women awkwardly trying to pitch and throw while another stands nearby fixing her hat.

The use of humor to demean female baseball players was a tactic commonly employed by opponents of women's suffrage. Reporters frequently portrayed women's rights activists as either masculinized or overly concerned with their physical appearance to the point that it prevented them from giving proper attention to serious matters (like casting an informed vote). As the structure and cultural creed of baseball began to take on a masculinized reputation, more and more commentators sought to discount or diminish women's relationship to the sport.

Even as the narratives around baseball as a male game took hold, female teams still grew in number as the game grew in popularity through the 1880s and 90s. Baseball, today still referred to as "the national pastime," became increasingly popular nationwide at this time and its sponsors and the press began to tie it directly to the American nationalist themes of the day. Influential proponents of the game like player and manager John Montgomery Ward pushed narratives that promoted an ideal white male baseball player, distanced baseball from similar games like Cricket that were popular overseas, and intentionally downplayed female participation in the sport. The "Doubleday myth," created whole-cloth in 1909 that tied the founding of baseball to Union General Abner Doubleday, is a prime example of this phenomenon.

The sports media employed similar tropes when mocking both female and black players, painting them as amateurish novelties. Shattuck finds it difficult to track black female participation in the earliest days of the game, but female baseball was by no means exclusively white, and there are records of several professional all-black women's teams in the 1880s.

As gender stereotypes around female athleticism slowly evolved, in the 1890s female teams began to focus on a less theatrical, more athletic form of the sport. But ultimately the men's pro leagues were too powerful, the instincts of the press to fall back on stereotypes too ingrained, and the moneyed interests too intent on promoting the men's pro game for any pro women's teams to find lasting success. Women's college teams slowly fell out of favor over these years as well. Women did occasionally play on men's pro teams in the 20th century, including three women who played in the Negro Leagues in the 1950s, the only women to play in a "major" league.