Whitney Houston cut short her park concert because her voice was horrible. I mean it fucking sucked. And since everyone's entitled to some kind of excuse, The Daily News says…

Whitney Houston cut short her much-hyped comeback performance on “Good Morning America” Tuesday after apparently losing her voice.
The Grammy-winning diva got through just three songs from her new album “I Look to You” before apologizing to the 5,000-person crowd for overusing her famous pipes.
“I’m so sorry. I did ‘Oprah.’ I’ve been talking for so long. … I talked so much, my voice,” she said, trailing off.

Talking for an hour must have made her sound like an evil witch. Definitely. That's why you don't see singers and bands talking before or after the concert / performance.