The most popular theory is that Eidos Interactive (before the Square Enix acquisition) issued a Cease & Desist order. I.G.I.-2 had sold poorly compared to Hitman 2 . The theory suggests that Eidos feared a free, superior product would cannibalize a potential I.G.I.-3 (which has never materialized, ironically).
Because Project IGI: Deviance has not yet been released, reception is based entirely on demos, trailers, and developer communications. Early previews from gaming outlets have been cautiously optimistic: PROJECT.IGI-DEViANCE