You asked why it was called post rock, not what post rock is.
But anyways, it's heavily instrumental music characterized by standard rock instrumentation (drums, bass, guitar), sometimes supplemented by orchestral instruments or electronics. The song structure, rather than being the standard rock verse-chorus-bridge structure, is more focused on building timbre and texture over repeated musical motifs and extended song lengths. Vocals aren't common, and even in songs that have singing it is not the focus of the music, instead supplementing it.