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Because the Union Flag looks shit as anything but a flag. Same for most flags that aren't a simple geometric form, ignoring decoration.
RSA's flag as a garment would get away with the borders on its Pall (Y-stripe) if there were fewer colours. Tanzania's flag has borders on its Bend (diagonal stripe), but the borders are both yellow, dividing blue, black stripe and green.
The Union Flag compounds its excess of components by lacking symmetry, the red saltire being counterchanged. This means you can't use the same design front and back without having misalignment at the sides.
Why do BC get a fashion designer in every 4 years to make ugly kit, when Denmark can use the same classic design for decades? :-)