One of the central characteristics of hip-hop is the art of bragging. This doesn't go well with results-oriented political ideology. Never mind that many "conscious" rappers have extremely suspect philosophies. The form has often been a forum for a certain conservatism of sexism, homophobia, anti-Semitism, and irresponsible and dishonest historical narratives.
But even when the artist's moral compass is pure, "conscious" rap is often a road to wack music paved with good intentions. Creating real change requires a certain kind of selflessness that is anathema to being a halfway decent rapper.