Hi Ortwin,
I'll give you my reason and then suggest another, perhaps more common reason.
My reason: where I work, I can't install programs on my machine. That means I have to use something that doesn't affect the registry. KeyNote, which I used before I came across UR, fits this just fine. It fits on a memory stick and requires no modification of the admin layer of Windows.
Since UR doesn't import directly from KN, I have to go the Keynote-TreePad-UR transit route. But since you can't export to TreePad, I don't have a way to get my UR mods into Keynote.
It would be very helpful to have that. An alternative transit path would be acceptable if there is one that is relatively straightforward to use.
No - the perhaps more common reason: I'm betting lots more people still use Keynote than UR, in part because it's free. So envision someone collaborating with others, some who own UR and others who don't - and don't want to spend money on software. Giving a round-trip transit path enables the collaboration. Not having it makes things tougher.
I understand that when you go between software apps, you might lose things not supported by the other app (e.g., keywords, attributes, forms, etc.). I'm OK with that. I can limit my drafts to not making use of the more advanced URP functions and add them in the final work-product.
Hope this satisfies your curiosity - let me know if not.
Regards,
Bal