I think having English heritage is not trendy for several reasons.

• It's seen as the default (as you pointed out), and thus boring.
• It's not seen as exotic/fashionable because of stereotypes about the English.
• The English have traditionally been considered America's enemies because of what happened two hundred and fifty years ago.
• Stories being passed down (and possible exaggerated) from earlier generations about how the English oppressed their ancestors.

ETA: Man, you really riled up some people!