May be the kimono Japan’s OFFICIAL national dress?
Or is it just regular?

Traditional.
Specifically for wedding ceremony, festival, new year, and so on.

There isn’t any official thing in kimono. It is just customary.

a native Japanese

Thre is no such issue as the “English royal family”.

They are the British royal loved ones. England is only one part of our nation.

The Scottish and English monarchies merged in 1603 when King James VI of Scotland inherited the English throne.

It is proud tradition that goes back centuries. It is like saying: why have your presidents been living in the same residence for 200 years? The British have constantly been pretty huge on pomp & ceremony, so they continue to practice it.

Some of the ceremonies are from the turn of the century (19th-20th) but many are older. Its all about continuation and traditions. Just like re-inactments but its still actually element of how the government functions.