It was probably because they're not doing the merger in all their markets (at least any time soon), and they need to maintain the old platform and functionality

It was probably because they're not doing the merger in all their markets (at least any time soon), and they need to maintain the old platform and functionality outside the US as a result. Subsequently having two separate codebase was probably considered preferable to development on one slowing to a crawl while it chased after two diverging requirements lists.