Generally because I'd rather use RISC OS than Linux on a machine like that, where power is not that important and my technical demands would be much less. I'd compare it to my preference to use a BlackBerry over a PocketPC, the PocketPC is technically more powerful in every way, but the BB's usability trumps it every time.
On my desktop, power matters because I'm doing stuff like compiling code which would take the rest of my life on an ARM processor, but on a portable sub-notebook, I'm doing different tasks and have other priorities like battery life, usability, running cool on my lap etc.
If I paid a £100 premium to run RISC OS instead of Windows CE (which is what the ARM notebooks tend to run), then that would be fine with me. On a desktop, you'd have to pay me to trade in my Mac for RISC OS, and it would need to be a lot more than £100.