Likely because you could, in most normal conversational speaking cases. With T9 you didn’t have to press the number multiple times unless it wasn’t picking up the word you needed but at a high rate of correct predictability, only pressing 10 different buttons would be a lot faster than typing on the whole keyboard. The software being fast enough to keep up is a little less likely compared to modern smartphones but hey, at least you could send a confidently correct text with your phone in your pocket.