It's a story of lore (lol). Witches of Izalith used to cast flame sorceries, but it was forgotten long ago (it is on the description of DS1 staff of Izalith), so I guess in the cinematic, they are casting fire sorceries and not pyromancies. And this is why this weapon cast sorceries and not pyromancies. Also, fire sorcery is a gift of the witch of Izalith to her daughter, while pyromancy is a creation of Quelana, one of them (and the only one that didn't become a 50% demon 50% naked lady thing). Would have been cool to get at least one of those old spell during of one the games. But well, it isn't forgotten for no reason, I suppose.