Building from scratch means I can "design" everything the way I want to. And I can thus build it in such a way that the features I want to include are easily possible. As in the end I also want the app to be able to be used in combination with the SDK (so the Pebble SDK running on a PC or cloudpebble can send commands to the Pebble app running on SFOS to communicate with the Pebble watchapp).

And properly integrating with the timeline also would have been a lot of work in Smokus Pebble app, and the same goes for notifications (as the solution chosen in Smokus app doesn't allow to update notifications, nor removing them when the notification gets removed on the phone). While rebuilding things like music control and call support are neither complex, nor that time consuming.

And besides the technical reasons it's also a political reason. As Smoku has stopped development (although he's still merging the pull requests) I didn't want to rely too much on him and his app. And as he also has the closed source firmware upgrade support which is only available after donating that could become an issue.