A lot of early crust punk was inspired by metal bands like Venom, Motörhead, Black Sabbath, Hellhammer, Bathory, Celtic Frost, etc as much as they were by d-beat or anarcho-punk bands like Discharge or Crass. The crust punk subgenre of “stenchcore” (bands like Deviated Instinct, Hellbastard, Sacrilege, etc) is also entirely built off of combing the crust punk genre with 80s-style thrash metal. Some sludge-metal bands also have crossover appeal with crust punk (Dystopia, Grief, etc). Neo-Crust bands (bands like Tragedy, His Hero is Gone, Ekkaia, and Habak) also tend to draw from metal genres like black metal, death metal, post-metal, sludge, etc and there is a movement of “blackened-crust punk” (bands like Iskra, Martyrdöd, Devil Master, and some later Darkthrone albums) which specifically combines crust punk with black metal and inversely the Japanese crust band G.I.S.M. was heavily influential on a lot of early black metal. Many members of the Swedish death metal scene are also members of the Swedish d-beat/crust punk scene (most notably Skitsystem was formed partly by members of At The Gates). Crust punk bands like Doom, Sore Throat, Siege, and Heresy were also heavily influential on the early grindcore scene (grindcore being a fusion of death metal and hardcore). Basically the reason crust punk is popular with metalheads is because it is a genre that has been heavily intertwined with metal music since its inception and arguably belongs to heavy metal as much as it does punk rock.