Because perfection is the enemy of the "good enough," and iPhone + Android covers an enormous share of the market. Should they support Lynx on SunOS 5, too? No? Why? Because a vanishingly small number of people use that platform.
The app is simple enough that it seems within the realm of possibility that it would take less time to build it using the web thereby getting support for many more operating systems for free.
But that is backwards for this case. They are trying for "perfection" by writing a mobile app so it looks just so and the ui is just the way they want it, when "good enough" would be a simple web site that took a couple hours to make and have online.