If by "keyboad with T9 layout", you mean keyboard with ten or twelve keys, the reason why Qwerty layout is preferred is simple: it's easier and more intuitive to type text with a full keyboard rather than ten-keys one. Just like it's easier to type using ten keys rather than one (which is still possible, using Morse code).
If ten or twelve keys keyboard was (is?) so popular in mobile phones, it's because those phones were originally small to accommodate a full keyboard (count 26 letters for English, or more for some other languages, plus 10 keys for digits, plus many other keys for punctuation) keeping the size of each key reasonable.
Today, large tactile screens make this limitation obsolete, since there is more place for a keyboard, and you can display some keys or others depending on the context (i.e. you don't need, for example, to display permanently the digits, but only show them on request or in a given context).
As for auto-completion, nothing forbids someone to implement one in a full keyboard too, in replacement of T9.