What's exactly meant by the "slippery when wet" report on ATIS?
why the runway condition is expressed as "slippery when wet" rather than just as "slippery."


Because the runway is normally not slippery, and thus it cannot be described simply as "slippery", but it must be specified when this occurs, particularly when it is wet it is slippery.

Saying only "slippery" would mean that the runway is always slippery, and is not the case.

Saying only "wet" does not convey the information that it is also slippery in that condition. (and you want to specify this because not all runways are slippery when wet)

It is not said "slippery when icy" because "when icy" everything is slippery, so that does not need specification, "icy" is sufficient.

"slippery when snowy" I never heard of it, generally because either the runway gets snowploughed fast enough, or because when there is too much snow the runway gets closed.