I use both actually- not proud to say that I have spent 4 hours on the phone before several times. I maintain and operate radios in the military and know first hand why people prefer communicating via data rather than voice:
1. People can be hard to understand.
2. Some people can't gather their thoughts. You probably had a 10 minute phone call turn into a 2 hour phone call at one point.
3. Data is nice. If you forget, you refer to the text. It also lowers the chances of any error for the sender and the receiver since they can read what they write before sending it. If you have to take long reports or record the conversation via voice, it can be a pain.
4. You can stay in contact with multiple people at once using text. Can't really do it on a phone (skype doesn't count).