Probably because few are Americans.
Yes there was Borg in the 70s, but we had Connors, Ashe and McEnroe.
In the 80s we had Agassi and Sampras (ok Sampras wasn't the most charismatic fellow). But since then, there really hasn't been any American men to speak of in tennis, and with the exception of the Williams' sisters, none in the women's game.
A lot of the new stars in women's tennis are from Eastern Europe--training at tennis academies in central to southern Florida. People forget there is a large expatriate Eastern European community in southern Florida, and as a result many of those Eastern European stars are better known than it normally should be.