While it started with kids IMing each other, it was used because they tried to respond quickly. When texting came along. at something like 255 characters per text @ 25¢ a text, it actually cost more to type words out. Kids were already being ragged about $200 phone bills because of their texting. Then some kid saw the ad, "F U CN RD THS, U CN HV A GUD $$$ JOB" and the rest is history.
Yeah, texts are unlimited now, but old habits die hard.