She doesn't want another woman like Fauxchahontas to steal her spotlight.
Via NY Times:
Democrats close to Hillary Clinton’s presidential campaign signaled strongly Thursday that she would choose Senator Tim Kaine of Virginia as her running mate, rounding out the ticket with a popular politician from a battleground state.
Both former President Bill Clinton and the White House have expressed their support for Mr. Kaine, but aides cautioned that Mrs. Clinton had not yet made a final decision and that other candidates were still under consideration.