Part of the reason for kepub originally was support for vertical (vs left to right) languages (i.e. Japanese which was a key market for Kobo). This is stretching beyond my detailed knowledge, but roughly speaking the epub 2 standard doesn't support that and is what the adobe renderer included is based on. Kepub is close the epub 3 standard instead and it allows vertical languages. Since they were going to have to make their own renderer anyway, they probably figured (at least in part) that it was easier to put everything they wanted into it rather than split development between the 2 different renderers.
And I'm in the same boat that I know just enough to be dangerous
