The main cause of differences in rules between sports played indoors versus outdoors is primarily weather.
Outdoor sports are often designed to account for various weather conditions, such as wind, rain, and temperature, which can significantly affect gameplay. Indoor sports, on the other hand, are played in controlled environments that eliminate these variables, allowing for a more consistent playing experience.
While equipment type, game length, and number of players can also contribute to the differences in rules, they are generally adaptations to the specific environment rather than the main cause.