I think it really has to do with the customer base. I've seen many, many posts on other forums from people using DSLR's in aperture mode who can't grasp the idea that aperture and shutter are linked. They think they can close down the aperture and not get blurred subjects. If they only had shutter priority, they'd have more sharp photos.
It takes experience to know how aperture affects an image, and people buying cameras for their auto functions don't have that, so it's kind of a waste to offer that feature. It takes far less experience and knowledge to figure out that a slow shutter blurs movement.
I don't buy the argument that auto control of a mechanical shutter was too difficult. As has been said, they do exist.
Ultimately, while on one level I wish my Canonet had aperture priority, I don't think it would be very useful. As it is, I set the shutter, the camera tells me what aperture it's going to use, and I can adjust from there if I want. I can freely set the shutter for the subject movement and leave it. With AP, assuming it told me which shutter it'd use, I'd be no better off, and would have to always check to see if the shutter was fast enough. I might as well shoot in manual mode if I'm going to be manipulating the aperture for effect.