I think it's because Buffy actually shaped a lot of television shows with quirky heroines, or heroines with special powers. We wouldn't have Veronica Mars, or Lost Girl, or Nikita, or Vampire Diaries, or even Supernatural (not that it has girl heroes, but I suppose Jo and Bella owe their existences to Buffy inspiration, too), to name but a very very few, as long running TV shows without having had Buffy first.

As a matter of fact, I think even teen sitcoms changed because of Buffy - they used to always have a male lead, but now, the lead is much more often female. I know there are college courses in feminism dedicated to Buffy studies.

She changed the world. A lot.