Pipe Insulation Slit Orientation
Because there will still be a tiny bit of air access and thus water condensation.  You want the insulation acting like an inverted bucket, not a bucket that will catch the water and hold it against the pipe.  Catching the water will cause metal corrosion, mold, and waterlog the insulation.

Making the insulation heavier will also tend to make it want to fall downward.  If the top of the insulation is a crack, it will tend to want to yield and slip out.  Whereas if the crack is at the bottom, the insulation is "just sitting" on the pipe.