Oooh, some "why" speculation. My go, in order of priority (IMHO):

1. They wanted a device which a smaller physical size, but without compromising on screen size (same for moving the speakers to the sides)
2. The sliding keyboard hardware made it an obvious choice for the location.
3. A strategy decision was taken to be more finger oriented (thus making the tablet easier to use in moving vehicles/whilst walking etc., allowing for more visually appealing/less desktopey UIs and so on), and the dpad and menu key were less useful for that.
4. It allows for the exterior of the device to be more aesthetically clean.
5. Microsoft and space-based humanoid lizards are in a secret conspiracy and delivered hypnotoads to Nokia to destroy your brain

Honestly, I think it was (1). Whatever the reason, the smaller form factor of the N810 is much more preferable to me than the bulkier N800.