Because just the Bolt inside moves, this is the why the all the Mark series 22 are so AWESOME!!!!!!! the 22/45 Mk.III Being My favorite
A fixed barrel w/ a moving internal bolt, is by far more accurate, the sights are fixed and They DON'T MOVE when the pistol is cycled, so all follow up shots are very accurate, and quick, less waiting time for a follow up shot, Less mass moving.
Less felt recoil, or maybe it is that they feel like they are much better balanced
With a movable slide ala SR22P, and center fire pistols the sights move with every shot taken, follow up accurate shots take a slight amount more time, to find target.
More felt recoil