It's just tradition. Neither system is objectively better than the other, so you just have to get used to using both and switching between them from time to time!
Right-handed coordinates are traditional for modelling where one imagines the XY-plane to be horizontal, and the Z-axis to be vertical.
Left-handed coordinates are traditional for cameras where one imagines the XY-plane to be vertical (corresponding to screen coordinates if the camera is upright), and the Z-axis is into the screen (corresponding to the Z in the Z-buffer).
So modelling applications like Blender often use right-handed coordinates and game development toolkits like Unity often use left-handed coordinates. But some systems go the other way: RenderMan is left-handed, and OpenGL is right-handed (or at least early versions were, it's fairly agnostic now).
Another reason why handedness can differ is because of different conventions about the direction of the Y axis in two dimensions. Back in those far-off days when computer displays used cathode ray tubes, the display was scanned from top to bottom and left to right by the electron beam. So it was convenient to have the Y axis running down the screen, with the coordinate origin in the top left. On the other hand, it's conventional in mathematics for the Y axis to point upwards, with the coordinate origin in the bottom left. When you add a Z axis, these conventions give rise to three-dimensional coordinate systems of different handedness.