Actually, it's more common to see the country-code second level domain .co.uk in the UK
If you visit Blurtit.co.uk it will redirect you to the .com site though.
Why .com?
It's easier to remember.
And Blurtit was started back in 2006, when there was less competition for domain names, and so .com was the standard.
Also, although Blurtit is a UK-based company, this is the order of countries which we get most traffic from:
