Because she wants to stop medical treatment to die.
From the USA Today story:
Yet more than 1,000 people showed up for the party, with police and firefighters providing a motorcade. She received cards and gifts from around the world and shout-outs from a number of celebrities.
Think about this: If Jerika had chosen to carry on, nobody outside her family and community would know about her struggles.
But as soon as she chooses to die, she becomes a celebrity.
Click here for more from Wesley J. Smith on National Review.