Because she/they made the complaint and prosecuted it. The onus is on the prosecution to prove beyond a reasonable doubt the defendant is guilty of the charges.

The defense need not nor should not be required to lift a finger unless their lawyers saw the need for it. The stand is always a huge risk and theres plenty of opportunity to poke holes in the case since the prosecution basically had to put her on the stand in order for the case to have any credibillity