The only reason she's the only girl left is because Mandisa left too early! If Mandisa were still in it Katharine would definitely be next female to go.
No, she didn't. She was a one-dimensional singer who put up two bad weeks in a row. She left when she earned her ticket out. It's just unfortunate that it doesn't always work like that and that people over-staying their ability (Covais, Pickler, etc.) sometimes manage to hang on for weeks for reasons other than their talent.