In simplest terms, because he lost that support. The irony is that it was he support of Khrushchev that likely doomed him, the Premier becoming wary of Zhukov's stature following the failed attempt at ousting him in June, 1957. Zhukov was too successful, basically, in ensuring Khrushchev remained in power, and thus put a target on his back as a potential threat himself.

I've written about Zhukov's two falls from grace in this earlier answer, which expands on this a bit more.