I'd also be keen to hear the exact reason, but the text message is probably going through layers of infrastructure and software, possibly even satellite, before appearing on a computer screen, whereas the shockwave is traveling at the speed of sound in water.
Wouldn't that be because the sound went straight from point A to point B, while the text message had to travel via carrier infrastructure? If I send a text message to a person two cubicles away, it does not go straight to their phone; but, via a cell tower. (Probably a satellite in the case of the Titan sub.)