I think mixed race people appear to be more interesting due to the mix of ethnicities and cultures, and that makes them more desirable.
The idea of being 'exotic' is quite appealing to most people, like there's this one white girl I know who simply insists that she is 1/32 Malaysian because otherwise she'd be white and cockney like most of the people in the area.\
Also, a lot of people seem more interested in me when I tell them I'm mixed race (Turkish, Welsh and West Indian - which is quite a mix!)