I think its a case of available local resources.
Let's face it, a tablet (even multi-touch) is not ideal for combination commands and just isn't responsive enough to make your time with things like CAD productive.

I don't think we're likely to see web-hosted apps like Microsoft Office or Adobe PhotoShop any time soon, basically because of poor broadband speeds in many countries and the speed at which browsers can run plugins to handle these apps.
Not to mention the money thing. Businesses would have to make something out of it, so I'd imagine a subcription would be required. I don't know if it's just me, but I'd rather have the disc that I can use whenever I need and keep it on the shelf, locked in my home, rather than have a cloud key that I could lose and forget if my computer goes kaput.