UI is surface area. Having 2 UIs vastly increases the surface area of the application, which means tons of testing. It also means all of your developers have to dedicate resources to remembering 2 or 3 versions of the UI, and to an extent the history of those changes.
There are also degrees to which it's more difficult to memorize two loosely related things than it is to memorize two completely separate ones, because your brain keeps trying to treat them as similar even when they are not.
Old UI support has zero cost to it.
Arbitrary new UIs have significant cost to them.