Toyota has really put a lot of effort into the Gen 11 Corolla. I think in North America, the Yaris has been too closely priced to the Corolla, didn't offer much of a fuel economy advantage, and felt like a much less substantial car for about the same money - so the Corolla became the choice for most buyers.
With this - it makes sense for Toyota not to invest in the Yaris. Re-badging the 2 gives them a product, but allows them to remain focused on the Corolla.