Maybe I just get tired clicking the keys in my phone whenever I try to send long messages. Also, it's a bit hard to formulate a message nowadays, since every single difference in the writing style of the message can mean a lot of things for some people. For example, when a person forgets to add a smiley at the end of his message, the receiver of the message sometimes assumes that the message sender is mad at him for some reason. You can't also text using uppercase letters since this will be misunderstood as the text message equivalent of shouting. There are just so many existing text messaging principles out there that you can't afford to make a single mistake in how you construct your message as this can serve as a cause for misunderstanding.
That's why I seldom engage in drawn-out text message conservations. That, and the fact that I'm saving up my mobile load. :)