...As example yesterday Nordic released the announced Win/Lin game "Book of the unwritting tales 2" for Windows and Linux on the same day...
With due respect, there is quite some difference between an adventure game using 2D graphics and an RPG making heavy use of 3D - not least the greater choice of middleware for cross-platform 2D games.
Larian might have had an easier time with Mac/Linux conversions if they had used the likes of Unity3D but it would likely have meant performance compromises (plus Unity might just not have been a viable option when D:OS started development). However the fact that a Linux port is
underway (and a proper one, not just a prettied-up Wine wrapper) should be considered a positive step, however (regretfully) long the delay in releasing it.