Who says? This game looks fucking great as is, and changing it to make it look very slightly better seems like it'd be a huge waste of time. Especially since as
Walter Victor points out, for continuity sake Jestur would have to re-render the entire game. Changing to HS2 without doing that you'd be left with a very jarring style change once you hit that point in a playthrough, which is never a good thing.
Is there some AVN rulebook somewhere I didn't know about that says you can't keep using the engine you've already successfully been using for years to make a great looking game once X number of years pass?