Apart from her great music of course, she found a way to attract attention to herself. She did something different than all the other artists who have just gone more and more naked in the past years and it payed off for her. She knows which buttons to push for people to talk about her
I don't love her but she is popular, because she was smart enough.
She started appearing all extravagant and kinda ridiculous, but ridiculous in a way to call people's attention, with the outfits and hair styles. That was smart. Then Poker Face lyrics are repetitive, so it gets on everyones mind, and then people got obsessed with that song they got interested on her looked up for her, found out her other repetetive/catchy songs and started to like her.
I totally forgot two more reasons!
1st - She wrote songs for Britney and she took advantage of that to be popular.
2nd - The Christina Aguilera all thing gave her lotta popularity!