I suspect the biggest reason is that, these days, most desktops are probably sold to business, and there's no Killer App for business where a touchscreen makes sense. The vast majority of interaction most people have with their PC at work is typing text and numbers. That, along with the inertia of businesses, who want to get their money's worth out of their current equipment if there's no good reason to upgrade, right now.
I haven't looked at detailed numbers, but from what I'm hearing, a lot of people are buying tablets to consume the the 'Net at home. If not a tablet, then mostly Netbooks and laptops. I think desktops at home are the minority, right now.
Part of the slow uptake of desktop touchscreens is a usability issue. Back when touchscreens were first being introduced I read the term "Gorilla Arm," describing how people feel like their arm/shoulder is getting a workout from having to hover their arm over the desk in order to touch the screen. Depending on how the desk is arranged, it's terribly inconvenient to use a touchscreen.
I'm a gamer, and a programmer, so I have desktops at home. I have a deep desk, 3 ft deep, with my monitors positioned at the back of my desk. Depending on whether I'm leaned forward or back, my eyes are literally 4 to 5 feet from my monitors. So, I can't reach my monitors unless I go out of my way to lean way forward.
Besides being a gamer, I'm a web developer and I want to get back into game programming. As above, the vast majority of my work is typing. The interactions I need to make with the UI are handled quite well by a mouse or trackball. In my day-to-day work, a touchscreen is useless to me.
A friend of mine had a Surface 3 for about six months, and I used it a coupla times a week. The touchscreen didn't excite me very much. I did find myself reaching up to touch it, occasionally, to click a link or scroll. But, most of the time, I'm sure mostly out of habit, I used the keyboard and touchpad. Since I was often using the device while sitting on a couch or lying on a bed, literally in my lap, the lack of a laptop hinge for the screen made it extremely cumbersome to use. I didn't like it much at all.
On top of all that, there is the added cost you mentioned. Without a killer app to justify the added expense, I suspect most people will not spend the money.