She openly loved him back.
It was his first relationship. Meaning he was inexperienced in emotional attachments.
It ended Badly. (It didn't take much for Naraku to turn them against each other.)
Once burned, twice shy. He is afraid that declaring his love for Kagome will somehow result in her death as well, especially with Naraku still around.
It's obvious from his later encounters with Kikyo that he feels guilty for her death, for failing to protect her, and for allowing himself to hate her fifty years ago. For him, loving Kikyo happened yesterday. He never had closure with the live Kikyo and can't abandon her now. Plus he still hasn't matured enough to handle his emotional relationships as directly as he handles his battles. In many ways, Kagome is much more mature than he is.
Once the threat of Naraku is past, he may then be able to finally admit how he feels without the threat of it endangering Kagome.