I’ve fished since the mid 1960s with both conventional and spinning reels. I’m right-handed. A majority of the world is right-handed. If you are right-handed, you have more dexterity in that hand for cranking the reel. You don’t need any dexterity in your left hand to simply hold onto the rod. Yet, despite that right-handed conventional reels have the cranking handle on the right side, spinning reels by default have the handle on the left side. [And yes, I changed the handle side to all of my spinners.]