The main reason is viewfinder usability. In many burst-shooting situations—especially sports and wildlife—you do keep tracking the subject through the optical viewfinder while panning. If the mirror stayed up for the whole burst, the finder would remain black and it would be much harder to keep a moving subject framed.

So, on most DSLRs, the mirror drops between frames to restore the optical view. That brief return of the image is a practical advantage even if it costs some speed and adds mirror movement.

That said, some pro DSLRs do offer a higher-speed burst mode with the mirror locked up between frames. In those modes, you lose normal continuous optical viewing, but the camera can shoot faster because it doesn’t have to cycle the mirror each time.

So the trade-off is:

• mirror cycles between shots: slower, but you can keep seeing the subject
• mirror stays up: faster and less mirror movement, but the optical finder blacks out during the burst