[simiones]

You're right about the first part (we don't really know why QWERTY was designed the way that it was), but as far as I understand it's pretty clear that Dvorak was created to enhance typing speed and comfort, based on ergonomic principles not related to mechanical details. For example, it is designed to alternate typing between the two hands for most common letter combinations, and to have most common letters on the home row.

[theonemind]

I've heard that that the letters for 'typewriter' all appear in the top row so a salesman could type that as a demonstration. Perhaps apocryphal.