Because she's not a Special Prosecutor.
On 23 March Norm Wolfinger, States Attorney for the Eighteenth Judicial Circuit Court of Florida asked to be recused in the Trayvon Martin shooting case. To replace him, the Governor nominated the States Attorney for the Fourth Judicial Circuit Court, Angela Corey. For this case she is just the prosecutor.
She has a duty to the people toinvestigate the first DA who found no crime where she subsequently did (without any new evidence). SOMEONE has this duty. Of course no one on the left is stupid enough to prosecute the DA because they know he would defend himself.