A380 reverse thrust on only half the engines I noticed on this video of an A380 landing in rain that reverse thrust was applied to only the inboard engines. Do the outboard engines not have reverse thrusters? If they don't, then why not? If they do, then why not use them on a slippery landing?
Yes, the outboard engines do not have reverse thrust to avoid them kicking up dirt. On a regular runway they will be outside of the concrete surface, so using reverse thrust there will incur the risk of foreign object damage.

This was already covered in this answer.