Why is it customary on Good Friday NOT to observe communion?
It isn't. There is communion on Good Friday.
There is no Mass, no sacrament, on Good Friday. The Church observes the memorial of Christ's death by being sacramentally dead from the climax of Holy Thursday's liturgy until the Easiter Vigil Mass.
SD
As Dave pointed out, there is communion, but it's the "Mass of the Presanctified" -- there is no consecration; the elements are those consecrated at Holy Thursday Mass.
The Mass is the unbloody re-presentation (to us) of Christ's sacrifice on Calvary. On Good Friday, our attention is entirely on the historical event of his passion and death 2000 years ago, and so we "fast" from the re-presentation of the sacrifice in our own time.
There is no Mass.
Reading of the Passion, Stations of the Cross, Sorrowful Mysterieis of the Rosary -- but no Mass. No Communion.
Good Friday is the ONE and ONLY day of the year when there is not a Mass.