Because TPTB+Blake decided that she's a perfect Team Blake member for this specific season. Kat is a decent singer for what she does, but I'm pretty sure TPTB could cast like 10 Kats every season with a minimum of search effort.
"Indie" people usually rely on a creative style, tone and charisma rather than vocal prowess. Kat's style is heavily influenced by Billy Joel and I personally think that this style doesn't fit her tone at all, and I don't find it original either. Now if we're talking about charisma, she's cute but not particularly memorable on stage.
In short, she is in the right place at the right time. This might sound harsh, but hey, Kat is still in the competition, there's plenty of time to prove me wrong :D. JMHO :flowers:.