what's the meaning though behind why hyphenated names are popular names in the UK and not in the USA? Is it just a UK thing or are they also popular in Canada/Australia/New Zealand?
The Brit aspect, I believe, was used mostly when two (or more) "estates" or "landed nobility" were joined in one union. Often, I think, it had to do with a family line's name ending due to only daughters being born to the last male heir. It was a way to keep two (or more, one family has like 5 hyphened last names) fancy names alive - and some like Helen Bonham Carter skip the hyphen.) There are a lot of Smith and Jones hyphens too, I think, just to keep them straight/differentiate. Not as common in the US, though now they are becoming more common, I would guess. The Spanish have lots of double surnames too, but that's a different tradition.
The Dutch do it their own way, as in ex-Celtic and Hull City striker Jan Vennegor of Hesselink, whose last names were the union of two prominent families.