I believe one of the main motivations for releasing a digital download version was so the game would be available to people new to the Divinity series, brought in by D2:ED.
Distributors may have had a problem with an updated version of the game if they knew the core fan base would not be potential customers.

Larian has released several patches for all of their games. In this case they may have put enough effort into the update that they consider it more than just a patch, or they would love to provide it as a patch but couldn't get download distributors interested doing so, or providing a patch could just be more hassle than they want to spend on this while they are still working on D2:ED.
If you don't think the downloadable version is worth it, don't buy it.

Funnily enough I've read on other forums how Windows 7 should have been a patch for Vista, Left for Dead 2 should have been a patch for Left for Dead...

All downloadable editions of the game are the remastered version.