Why are haiku based on nature?

Not all haiku are based on nature; some classical haiku deal with other themes, but nature seems to be a common, universal theme that transcends linguistic, regional and cultural barriers and can be appreciated by others on its face without having to resort to explanation:

Leaves fall in autumn

only to return in spring

full of life again

Can be understood by anyone who lives in a place with deciduous trees (and speaks English).

However, something regional like:

Did the bridge fall down?

The one in Old Londontown.

Wish I could see it.

Requires not only knowledge of London bridge, but also of the nursery rhyme and of its historical vs. modern appearance, limiting the number of people who can appreciate its full "story".