As Jock mentioned earlier, the kilt is first and foremost the national dress of Scotland.

For some people (regardless of ethnic background) it is a uniform, such as in some military regiments or in pipe bands. For others it has developed into a symbol of pan-Celtic identity, especially in the New World. This is a more recent development but Irish nationalists began promoting it in Ireland around the turn of the 20th century.

The kilt is also a stylish and comfortable piece of clothing, which is why they come in many varieties from full Highland regalia to non-traditional looking garments such as a Utilikilt.

A better question might be: why not wear a kilt? Answer that, then proceed to wear it at every other opportunity you like!