Watts and Charles Wesley both wrote near-epic-length poems. Hymn selection committees tended to pick a few verses for each particular hymn. Often you will find that the verses for several different hymns actually came from the same original long poem. A quick trip through, say, Southern Harmony, with Google at your elbow, will confirm this.
Also, "common meter" i.e. 8.6.8.6 tends to produce the zipper verse effect just like in folk songs.
-glenn