^ Blake's character Serena is nicer and friendlier than Leighton's character. Leighton/Blair often comes off as intimidating as she plays a ***** on the show. So it's natural that the American audience has warmed up to Blake more. Also she was quite famous prior to Gossp Girl and starred in the hit movie The Sisterhood of the Travelling Pants. Blake has that awkward, carefree appeal that the American audience loves.
The American audience isn't responsible for putting her on magazine covers. Editors are. For whatever reason the editors have made Blake the chosen one.