The song is in D minor, but that chord first comes in at the 7th beat of the 16 bar progression. So when the song ends cold on the first note of that progression, it ends on Bb. This gives the listener a subtle feeling of an unfinished song, even though it ended on the 1st beat, which is typical of most songs. By not resolving the chord, the listener is more apt to hum the song and therefore more likely to need to listen to it again.
--Why Ke$ha went #1... and why it could've been bigger.... Also, chick looks like this: