One word: ergonomics.

With Dvorak, the most commonly-used letters are on the home row. In fact, all of the vowels are on the home row.

The most common letter in English is E, and QWERTY makes you reach for it. You know what IS on the home row? Fuckin’ semicolon! Can you even remember the last time you used a semicolon?

I spend a lot of time at work sitting, so I’d rather have a comfortable chair than an uncomfortable chair. Same with my keyboard: I’d rather have a comfortable layout than an uncomfortable one. Less risk of repetitive strain injury, too.