The Union Jack—officially the Union Flag—combines the crosses of three patron saints: St. George (England), St. Andrew (Scotland), and St. Patrick (Ireland). Wales' absence isn't an oversight; it reflects 16th-century constitutional reality.
Wales had been fully incorporated into England since the Laws in Wales Acts 1535–1542 under Henry VIII. It wasn't a separate kingdom or principality with distinct international legal personality—it was England administratively, legally, and politically. The English flag already represented Wales.
The red dragon (Y Ddraig Goch) symbolizes Wales culturally, but flags in 1606/1801 represented sovereign entities entering union. Wales had no separate sovereignty to contribute. The Principality of Wales was a heir's title, not a state.
Devolution (1999) created the Senedd (Welsh Parliament), but the UK remains a unitary state. Proposals to add the dragon have been raised (2007, 2013) but rejected—partly because the flag represents the union of crowns, not a federation of nations. The Royal Standard (monarch's personal flag) does include Welsh heraldry in some versions.
St. Patrick's cross represents Ireland historically, not Northern Ireland specifically. The 1801 union was with the whole island; partition (1921) left the flag unchanged.