Who knows? In Optimum’s case they may have found it too difficult to recreate the choir of children singing with the handful of female VA’s they use for child characters and just left it as is.
In VIZ case they don’t dub all the songs to begin with only the ones where the characters are actually singing. Even though 3 o’fairy should fall into the latter perhaps VIZ regarded it as an insert song.
Optimum I'm willing to cut credit for, but Viz really has no excuse. This is supposed to be the definitive English adaption of Sailor Moon, but I guess they care more about having Haruka and Michiru's romantic feelings known (even though their latest booklet failed to do this) than dubbing these songs. A real letdown.