We generally think of volcanic eruptions as flows of lava, which is just melted rock. Pompeii was buried by small stones and ashes. To the west of Pompeii, Ercolano (called "Herculaneum" by the British) was simultaneously buried with hot mud. Digging Pompeii out of the loose dirt was a simple task compared to chiseling and drilling Ercolano out of solid rock.
Of the two archeological sites, I find Pompeii more interesting because it was a typical Roman town where there lived both rich and poor. Citizens of Ercolano were more affluent than the average resident of the Roman Empire.