For me it has a lot to do with work. I work in an industry where anything can happen at anytime and then I have to 100% focus on what's going on. Many times this makes phone calls impossible as I would have to hang up with someone 6 times during the conversation. So for me txt'ing is easier and faster and its something I can do at the same time I'm doing something else.
Its all about multi-tasking for me. In fact right now I'm txt'ing Kayak Rat while writing this and while working