Because it's f**king boring that half a dozen countries around the world still have variations of the Union Jack as their flags. It's much better for every country's to have its unique flag.
Also, every relic of the British Empire must be torn down.
That's not even a remotely good reason.
'This flag, unlike the previous flag, can readily be distinguished from other flags' isn't a good reason?
I mean, I can readily distinguish it just fine.
Ideally a flag should be distinguishable when hanging at all wind speeds..
Btw if you want to see an ugly Union Jack themed flag, look at Hawaii's for a particularly unappealing UK/USA/Yugoslavia hybrid.