The why is simple. Unigine is an engine that fits the scope of the game. UNIGINE2 is an engine that can create amazing virtual enviroments in a celestial scope. Unreal is more of a "pretty looking but contained" kind of engine. What the UNIGINE lacks in graphics (something that is irrelevant when it comes to gameplay) it gains in size. You simply can't have the level of tesselation UNIGINE provides in Unreal.
Also, UNIGINE has some pretty neat tricks folded within it, that it makes it suitable for Aerospace Industry simulations. Let that sink in. You know an engine is good on its complex calculations, when actual, real-life engineers trust on its projected simulations Bonus points for being russian Aerospace engineers,
I hope this answer covers your curiosity. UNIGINE2 is perfect for DU's scope.
