The basic layout of the MLB diamond has been little changed since the original Knickerbocker Rules of the 1840s. The distance between bases was already established as 90 feet, which was determined through trial and error is an appropriate balance between hitting and fielding. It provides frequent tests between the speed of a batter-runner and the throwing arm of a fielder.
The pitching distance, and other aspects of the pitcher's mound, and of pitching itself, have been tinkered with from time to time over the many decades to keep an appropriate balance between pitching and hitting.
In contrast to the distance between the bases, which seems natural, the very specific pitching distance of 60 feet 6 inches is one of those sports oddities which seems like a mistake unless you know the history of baseball:
The original Knickerbocker Rules did not exactly specify the pitching distance.
By the time major league baseball began in the 1870s, the pitcher was compelled to pitch from within a "box" whose front edge was 45 feet from the "point" of home plate. Although he had to release the ball before crossing the line, as with bowlers in cricket, he also had to start his delivery from within the box; he could not run in from the field as bowlers do. Furthermore, he had to throw underhand. By the 1880s, pitchers had mastered the underhand delivery quite well. The year 1880 saw two perfect games within a week of each other.
In an attempt to "increase the batting", the front edge of the pitcher's box was moved back 5 feet in 1881, to 50 feet from home plate.
The size of the box was tinkered with over the next few years and pitchers were allowed to throw overhand starting in 1884. In 1887, the box was set at 4 feet wide and 5 1/2 feet deep, with the front edge still 50 feet from the plate, but the pitcher had to deliver the ball with his back foot at the 55 1/2 foot line of the box, which restricted his ability to "power" the ball with his overhand delivery to a certain extent.
In 1893, the box was replaced by the pitcher's plate, although the term "knocked out of the box" is still sometimes used when a pitcher is replaced for ineffectiveness. Exactly 5 feet was added to the point the pitcher had to toe, again "to increase the batting" (and hopefully to increase attendance, as fan interest had flagged somewhat) which resulted in the peculiar pitching distance of 60 1/2 feet.
Originally the pitcher threw from flat ground (as softball pitchers still do), but over time the mound was developed, tipping the balance back the pitchers' way somewhat.
The dirt path between the pitcher and the catcher was once as wide as the pitchers box and resembled the "pitch" area used in the game of cricket. Sometimes this path extended through the batting area and all the way to the backstop. Once the rounded pitcher's mound was developed, the path became more ornamental than practical, and was entirely abandoned by the late 1940s, although some recent teams (such as the Detroit Tigers) re-created the path in their new ballparks, for nostalgic reasons.
