Japanese culture has absorbed many Western customs due to globalization, etc. One is Christian weddings. But as you can see from these dresses, they don't necessarily resemble weddings as we know them. There are chapels set up all over the place because Christian weddings are photogenic, but it's more about wearing multiple outfits and being fashionable--most brides will wear a white dress or have a traditional Japanese wedding followed by a "wedding party" where she wears a second (and maybe a third) dress. As far as I can tell, the party dress isn't usually white and most women look like pink powder puffs instead of brides. Pretty pink powder puffs, but still.
As a side note, I do really like this black dress, just not as a wedding dress.
In any case, that's what happens when cultures collide superficially.