Two reasons why I won't touch a Dvorak layout:

1. I'm already a decent QWERTY touch typist. Since both keyboards use the same physical arrangement, knowing two key layouts would bugger me up. (I once tried Dvorak on an ergonomic keyboard. It ended up that I couldn't use QWERTY on that keyboard if I tried because of my mind jumbling them up.)

2. Everyone uses QWERTY anyway. Using Dvorak would either mean carting my own keyboard around, or finding somebody with administrative access to change the keymap.