I have to agree with other comments here. The tablet form factor is not "winning" per say, just showing that technology has progressed to a point where alternate form factors are now viable for different situations. Tablets are selling like hotcakes because the market hasn't reached saturation point yet. For the most part typical keyboard/mouse style pcs have already filled the market so they are just selling at a standard new user /replacement rate. Tablets are a new form factor so that market is not yet saturated. I have 2 tablets now and I only use them for content consumption when I want to be away from the keyboard. I still find them frustrating to use though for content creation (edit: ...involving any serious degree of typing). This was typed on a touchpad but this is about the limit of what I would ever want to write on a tablet. I won't touch on the numerous ergonomic problems as others here have done that, but they are many.
I think the more correct statement is "Why the tablet form factor is more popular".
I don't think it's necessarily going to win, and over the long haul, I think there are ergonomic issues with it. If you hold it flat on your lap, then you don't get a straight-on view of the tablet. And typing is horrendous, and hurts my back after about 30 mins. This is the #1 issue for me.
Sure, you can stick it on a stand and attach a keyboard... but then now you have a desktop (not even a laptop, since the keyboard is built it). There are plenty of applications where it works, like most especially reading ebooks, where you don't need to read and type at the same time. Also, if you can incorporate a writing instrument with it and you can write on the tablet, or playing board games, etc.
So it's definitely popular right now because it's cool and the designs are pretty sweet.
But if you intend to work on it, ie. type and read at the same time, I think over time, serious ergonomic issues are going to turn up.
The part being left out is this: we've had only one setup for computer users since (what?) the 1960s and it would be incredibly surprising if that was the way everyone wanted to use computers all of the time. Should we really be surprised that the basic setup of monitor + keyboard isn't the only or preferred way to use a computer?
After all, it was just a way to combine some existing technologies to improve on the paper-based user interface. It's not like it came down to us punched on golden cards as the perfect way.
I've used the HP Touchpad for a week now and I have to disagree with his claim that the ergonomics are better. OSHA says that screens should be at eye level. If you're sitting up straight, there's no way a tablet will be at that level. Tablets requires the user to be looking down at his lap constantly.
And if you're not sitting up straight, then your back will complain bitterly, if not now, then a few years later.
Regarding posture, I use my tablet mostly like other people hold their book or magazine. At least it can't be less healthy than what those oldfashioned people are doing (in reasonable lighting).
holding the bigger 10" tablets with one hand becomes tiring. If you lay it flat on a surface then you become hunched over. If you prop it up then typing anything becomes a pain, and having a smaller laptop becomes preferable at this point.
The tablet form factor is not "winning" The iPad is winning. While the form factor and natural interface is a large part of this, it is not the only reason. The closed ecosystem of iOS is big part of the success too. iPads are computers for 'everyone else' and by that I mean that you don't have to worry about malware, viruses etc: Your grandparents and my grandparents can make almost full use of these devices without constantly needing help from the likes of us.
It has nothing to do with iPads being trendy. Real people see them and immediately understand them. They try them and they want one too - here is the perfect device for sitting on a sofa and browsing Facebook while watching TV. They are also perfect for keeping young children occupied in a safe environment for 20 mins while you sort out your chores. This is not the case with an internet connected PC (or Mac for that matter).
iPads are not going away. They are not a 'trend' (by which I mean they are not a blip on a graph). Expect them to grow exponentially and for very household to get at least one.
Eventually, some other company will work out that it isn't just the form and the touch interface. The accessibility to a wide demographic, the safe sandbox, the vetted apps, the music+video+app+book store are all vital.
Whether it is Google, Amazon or Microsoft (or another) that finally crack this remains to be seen.
They are currently successful because they are "cool".
People have not realised that they are not that practical yet when you have to actually do something important or useful (other than consume at the vendors' whim).
Ask a typical tablet user what happens after about 6 months. It gets used less and less when people hit a productivity brick wall. I've witnessed this happen in at least 10 different people now back from the day of:
"Wow look at me, I've got a new IBM X61 tablet" ... 6 months later ... "I've lost the pen and just use it as a normal laptop now"
-to-
"OMG ipad rules - I can do everything on it" ... to 6 months later ... "I'm using my laptop now because it makes my hands hurt and I cant get it in the right position ever."
Different form factors, different uses.