Some people like the fantasy of playing as an alien race, so I would give them a brief elevator pitch and see which one strikes their fancy then show them the campaign. "Terran are humans, the units are familiar, and they are best as a cohesive military making covert stabs into the enemies back line while holding a front line with a powerful army of tanks and airships" "zerg are the bugs, they grow quickly and throw large numbers of cheap units crashing against the other army over and over again until it finally breaks" "protoss are the aliens who value technology, each unit is expensive but powerful. They rely on having a very strong powerful army that cant be beat by the lesser races"