Short answer? Because it's useful.
Long answer: after O'Neil brought her back in Question, she was used by Starlin as a full-fledged evil villainess in Death in the Family. And as it so happens with these things, more people were (and still are) reading that instead of Question. This characterization was further compounded by Dixon's Shiva, who appeared in the first Robin miniseries and later in Knightsend, always as a murderous trainer for the Batfamily, always with the "Ok, now you must kill to prove your worth!" shit.
Both writers used Shiva as more of a tool to advance the plot and to train other characters rather than explore her like the last half of Question V1 did. So the end result is that, for people who only read Bat-titles, that was all she was: an amoral or downright evil kung fu bitch working for the highest bidder who exists only to kill.
Things got a little better with Puckett's Batgirl V1, where Shiva actually got to use her healing skills again and was shown to have a little more depth (although I'll argue that the death wish thing at the end is still terribly OOC), but then came Hush, and Hush fucked everything over by linking her to the League of Assassins.