She may not be the smartest or most talented person ... but she knows how to work the media.
She started as a model, and the combination of heiress and model attracted attention. She's made sure that she's never far from the media spotlight: magazines, movies, TV shows, TV news, parties and clubs (good for gossip), books, Internet videos (whether infamous or humorous) .... The media knows that a Paris Hilton story is good for ratings ... and she's willing to oblige by providing stories on a regular basis.
As they say, she's famous for being famous.