For one, there's a lot more on the page than the linear feed. And then each tweet has several actions you can take on it, a drop down menu, etc. What was a single click becomes two or more keystrokes. When you factor in the other views that are even less linear - settings, editing your profile, messaging - you run into a long tail of cases that are poorly suited to a keyboard alone. This tends to be the case: a hyper-focused UI may work a little better with a keyboard than with a mouse, but a mouse does a much better job supporting the wide range of different cases out there and giving users a way to explore and discover the UI.