I totally agree about the evolution of OSS and linux, ten/twelve years ago linux was frowned upon, and look at it now - the most versatile OS on the market.Look at it from this standpoint, every major phone release has hardware that can rival netbooks and ultra-portables, when the dual-core snapdragons hit later on, they may even be on par with low end notebooks. They contain cellular modems as well as wifi so you're constantly connected on a device that can fit in your pocket.
On the software side every major mobile operating system out there is constantly evolving and adding more and more features of that you can find on any PC/Linux/OSX box in the world. Albeit they're slimmed down versions, designed to be lightweight and functional on a smaller sized screen.
Add in google voice, a service that you can have your cell phone calls funneled into a single number or have your google voice number funneled into any other phone number you tell it to, plus you have skype. Installing Debian on a mobile device doesnt change the fact its still a cell phone the only it changes is how much this cell phone can do without restrictions.
Its the people who push software and hardware to there limits and into places they were never designed to be, that push the technology world and push the companies to do new things. There the pioneers who are trying to marry the inevitable before anyone else is ready. Those are the people who make companies like Google, Nokia, Intel, etc etc realize that there is potential in a risk, when there is a community ready to back them up. If it wasn't for the indie developers out there trying to minimalize linux to be used on cell phones we wouldn't have our beloved Android today.
You ask why, I'll answer because its inevitable. You can join the front lines or you can wait.
That was my driving point why go a step back instead of developing the Android and Maemo (btw, whoever is looking for the fully functional Debian on a mobile phone should check Nokia's Maemo initiative and N900).
A bit on my "step back" comment - you have to realize that the utilization of available resources is at the premium for the mobile devices - whereas hardware became more and more robust, compact and affordable, the battery life still remains to be desired - right, what's there for mobility if you have to be bound to a power outlet whenever you go, well in this case you are not going anywhere); so the point is, the full OS will drain our beloved phones like a vampire drains its victim of blood. That's where Android and other ported OS's come into place, they are especially designed to keep those precious battery charges as much as possible.
I am pretty sure in the coming years we will see more and more devices running the ported linux OS, whether it will be Android, Maemo or something else. I doubt it there will be a mobile device running full linux shebang with Gnome/KDE etc - unless it is just for the fun factor of it.