I think the answer is money. Porting VF4 to DC would cost more money than a regular port of the first Naomi or a standard develop of a DC game. With the system already dead, they saw in PS2 a more reliable source for the return of investment. If you see, in 2001, the last "high budget" DC titles, like Shenmue 2, Headhunter or Virtua Tennis 2 were games that started their development in 2000, or were created with technology that was already DC optimized. IÂ´ve always have wondered, there is no possibilities of Sega make public their DC era engines for everyone? May be with that we can see finally lotta of canceled games and maybe modified "DC versions" of modern games.