Double post, but it's because of an epiphany.
To become psionic, a creature/person/whatever needs to be mentally touched or "turned on" by another psionic creature. This can be benevolent, like a wise old master opening his student's mind to its own powers, or it can be malevolent, like a person just barely surviving a Mind Thrust, and recovering to discover their own psionic powers.
This helps limit the amount of psionics floating around, but still allows the option for psionic PC's. They are each special snowflakes, after all.