Well I am not the Dev but they decided to write a installer from scratch, they did not use Debian's installer due to it not being as user friendly. As to not using Ubiquity I would think it would be more of a pain to port it to Debian that write there own from scratch.
See I don't think that was the issue. The latest beta release of Aurora (formerly EEEbuntu) is now built on Debian as well and it uses the good old Ubuntu-like installer still...
~Jeff
You may be correct Jeff it may be they did not want to have allot of Ubuntu packages in the build. I have not directly ask them why, I am only giving speculation. I could ask them if you would like but too me the new installer is fine and I am sure they will refine it to be more user firendly in time.