I guess that they were thinking about the people who want to use the range of accessories that they produce that don't integrate with the old app, and also people who want to have a schematic of entire layout with feedback showing where the trains are, plus a larger layout the old app wasn't easy to use, needing to swap between multiple pages.
There are lots of reasons but ultimately it is your decision what you want to use, Roco made their decision and created the latest version of the app and stopped updating the old app. Personally I prefer the new app - especially on tablets and I can absolutely see the attraction for the generation that they are aiming their product at.