Because the explosion was *very* energetic - sending debris out much faster than we're able to send probes (like Voyager).
Voyager is about 16.5 light-hours from Earth; that's about
186,282 x 60 x 60 x 16.5 = 11,065,150,800 miles, and it's taken 42 years to get there. That's about 263,455,971 miles per yer.
5.5 light years is about 32,332,370,637,600 miles, so the Crab Nebula's outer shell of debris is moving about 32,332,370,637 miles per year.