My best guess would be because there are many less volatile solvents available that are fit/safe for human consumption and don't require specialized apparatus (pressure vessel) for the extraction process. Most members on this forum are hobbyist and lack the expertise required for using more exotic solvents/equipment/processes. Butane is a gas when not under pressure so the extraction process would require using a pressure vessel of sorts. I'm unsure if Butane would deposit (leave behind), any harmful/toxic residue but being a petroleum product (like Hexane), I'm relatively certain it's mildly toxic to humans. There are several extraction solvents I haven't used because they need to be completely removed/neutralized after the extraction process is complete. Denatured ethanol, isopropyl alcohol and Hexane are some examples.