In todays hard economic times, it quite simply falls back to the bottom line.
Its cheaper to do a film in 3d then to pay Big name actors anymore.

Look at the box office numbers, it isn't just the movie anymore that makes or breaks the gross earnings now days. Its the amount of the paycheck for the week it was released.

Voice actors are inherently cheaper then live action actors.
Even those who do both, don't make as much for voice acting as they do actual screen acting.

Also, Motion capture...pay some no name actor to act the part out on a blue screen, plug the motion capture into the modeling software for animation, render, bam.

Its all just broils down to the bottom line.
G-man