As mentioned, speeding the game up is the main reason.
Pro/OBR: 0ne per pitcher per inning.
NFHS: Three per game/seven innings. One per each extra innning, no carry over of unused visits from the three in extra innings.
NCAA: Three per game/seven or nine innings. One per each extra inning, any unused visits can carry over. Also a second visit in the same inning requires a change.
LL: Three visits per pitcher. The count resets for each pitcher. You can make all three visits in the same inning.