Maemo patches a *lot* of things to work in a very unique way, i.e. the optification you see for most of the base system that places caches and all sorts of stuff on the eMMC while just keeping the bare bones on NAND.. that's an awful lot of work to do initially and maintain for a single device's adaptation (as you'd never get upstream projects to accept this, because they don't need it)

plus it has a lot of annoyances/downsides, etc

further reading: http://wiki.maemo.org/Opt_Problem