Ask HN: Why does being in an accelerator pre-YC lessen your YC chances?

Our startup was in an accelerator previous to applying to YC, and this was the reason why we were rejected after the interview. I understand the concern that we should already be "accelerated" by now but its hard to compare the previous program we were in to the caliber of YC.

Why does being in an accelerator pre-YC lessen your YC chances? Has anyone had similar experiences with this? Are there any YC alumni that had their company in an accelerator pre-YC?