^^This.

Despite all the responsive design tools and declarative layouts to ensure a great layout even on an oddball resolution... the fact is that in practice it's nontrivial, and remember that the companies that make most of the apps you use have finite design and QA budgets. They need to actually use the resolution in question in order to really do a good job of designing for it, and giving them double the work, for a $2000 and up phone which probably won't be super highly adopted, is a recipe for getting apps that end up with layouts that aren't optimized properly on one (or both) of the screen sizes.