Why can plastic become charged if you rub it, but not metal?
Plastic is an insulator.
When you rub plastic, you transfer electrons from one material to the other. They are then stored in one material (making it negative) while their absence makes the other material positive. Because plastic is an insulator, the electrons cannot flow through it so they effectively get stuck there - they are static.
In a metal, the electrons would get conducted away (e.g. back to the material they have come from).
I hope this has helped; let me know if I can do anything else:)