<p>Most of the time in Harry's 6th year, we find Draco repairing the vanishing cabinet. The purpose was to get the deatheaters inside Hogwarts. If Dobby could apparate and disapparate in and out of Malfoy manor with Harry and the others, why couldn't Miss Bella or Miss Cissy take Kreacher's help and apparate inside the Hogwarts kitchens? </p>
Because Kreacher serve only Sirius and after him Harry, Kreacher once left the house when Sirius said get out, but he can't follow someone else's orders so the deatheaters can't order Kreacher to apparate them in Hogwarts.