Read into what Urasue says when her magic activates and Kagome's previous life comes bursting out.i. Kikyo's soul is mad with anger.
ii. [The soul holds] a powerful grudge.
iii. Something damnable must surely have happened to [Kagome] in her previous life.The third statement for me sums it up; Kikyo is a remnant of the vengeful feelings that Kagome still holds in her soul because Kikyo died without a chance to cleanse the hatred or anger.If you don't trust a witch's twisted logic, listen to what Kikyo, a priestess, says:i. My soul cannot shed the hatred.
ii. As long as the hatred is not erased, my (actually, Kagome's) soul will not return to [Kagome's] body.I guess the suffering is too deep that it remains even after the reincarnation. Kikyo is basically the pain given a physical body by Urasue's curse.