They made a conscious decision to not go after mainstream Hollywood talent this time. They needed it for credibility and marketing in the past but at this point the GTAIV marketing machine did just fine without Samuel J Jackson or Ray Liotta doing voices. Talented voice actors are a much better deal than A-list Hollywood talent for voice acting. The real shame is that voice actors are so underpaid by the market. Billy West, who voices Fry and many more on Futurama, has talked about this in various forms of media.