Some reasons already stated in this thread (including the songs' catchiness, sexy lyrics, etc.) are why the songs are so popular. In addition, they're popular because of the people who made them.
Sexyback was popular because it was catchy, but it was popular also because it was sung by Justin Timberlake, the heartthrob from *NSYNC.
London Bridge was popular because it was catchy as well, but it was cool also because Fergie, the main vocalist from Black Eyed Peas, sang it.
I think an artist's prominence and popularity has a lot to do with how popular their song(s) or album will be.
