An overal design goal is also to keep it as simple as possible.
First because it is more realistic that it gets developed some day.
Second it is easier to understand and use.

Open Transactions or Ripple are very advanced and complex solutions. It is even for tech people like here in the forum difficult to really understand it (and trust it). For non-tech savvy people it would be even more difficult. A simple system would help that people really use it and therefore create market liquidity.