>>212524322It was just so that when Deb finds out Dexter's secret, it would be even more impactful for her. Dex was built up in Deb's head as something he wasn't, to an even greater extent than just being adoptive siblings, so that for her it could come crashing down harder.
Thus, her attraction to him only becomes a thing toward the end of s6, and after the big reveal where she finds his slides it no longer serves a purpose and barely comes up again