Because I like designing 3d printers.
I made a deltabot, and it's really cool. I can print really tall things.
But 3d prints are weakest in Z - printing really tall is maximizing the weak axis.
So instead of maximizing Z, I though about maximizing Y. First concept here is the DoppelBot, which is a cube-shaped reprap that's easy to enclose, and has, by default, a one cubic foot build area. the print area of a standard reprap. It accomplishes this 'twice as big' by the simple expedient of using two heated beds.
The basic design uses two lasercut endcaps, joined by OpenBuilds V-Rail. The lasercut endcaps ensure that your rails are perfectly perpendicular and parallel - they don't even need to be cut to the exact correct length. The machine itself uses a CoreXY gantry, and a Z bed with 3 independent leadscrews - so that the bed can truly level itself, none of this bed-level-compensation, if your bed is flat this machine will be able to tram it.
I'm experimenting with 9mm belts and over-built 3d printed carriages, to increase rigidity.
Also experimenting with a continous build system, wherein you can print dozens of objects in a row without human intervention.
[[/images/complete_machine.jpg|Doppelbot running its first print]]