I have never sent a SMS on a cellphone in my life. But I am going to defend those who do often and have developed the common use of text speak. Within even a single generation we can see the speech and all communication patterns change. While the change is not normally very drastic it is however noticeable. Even more so if it is driven by technology.
Text speak I believe is a product of our current society. People for the most part live in a never ending rush today. When we expect something we have now expect it to be fast if not even instant. With that said the kids simply using the world they know and mixing it with what they do. We set them on this path and they evolved it as will their children.