Most likely because spearmint was short for Wrigley's spearmint which was trademarked. I don't think spearmint chewing gum was a generic term at the time. Simil

Most likely because spearmint was short for Wrigley's spearmint which was trademarked. I don't think spearmint chewing gum was a generic term at the time. Similar to how sellotape(TM) was trademarked but now cellotape is a generic term.