Why are accents sexy? Why is "exotic" sexy?
It may simply be novelty, but I'll make up a just so evolutionary story:
Gains in trade are extremely valuable - and are a great positive. However, the asymmetry of "hard come, easy go" [i.e. it is very hard to build things up, and very easy to smash them down] makes fear of outsiders and violence a prudent behaviour, a behaviour that is detrimental to trade.
However, if your tribal group becomes linked via family and marriage to another group the risks go down and the benefits of trade and exchange can be more easily achieved.
Thus, any group that found outsiders sexcellent would marry into other groups and gain the tools, ideas, and products of others in peaceful and mutually advantageous exchange. These advantages would lead to better survival odds and thus select for the trait of finding difference and other cultures sexy.
The legacy of this is our love of foreign accents and other "exotic" features.