They've not sold well because they've been cumbersome and have involved massive sacrifices. Compare typical phone models with a cheap laptop when it comes to raw computing power.

But that gap is rapidly diminishing - the phones are catching up.

And as for user experience, you need to be able to get a "proper" desktop OS, and you need to not have to plug in lots of cables. And it needs to be able to drive a sufficiently high resolution display at good speed. It's slowly getting there, but it's got a bit to go still (but consider that today, mid-range Chinese unknown brands churn out quad core Android tablets with 2560x1600, and even low end phones come with 1920x1080 screens; and many of these devices can handle 3840x2160 output to external screens...)