There are several reasons Witten isn't a great weapon in the red zone:
1. When not in the Red Zone, Witten gets open on 7 YD In/Outs because the WRs take the coverage 20+ yards down the field. Obviously this doesn't happen in the red zone since the field is shortened.
2. Witten doesn't excel and leaping for catches in traffic like Tony Gonzales...he is more of a 'square up in space' player meaning he needs to get open, turn and have his feet set. Nor is he good at running fades (and Tony can't throw them well).
3. He isn't a RAC guy who can catch the ball at the 5 yard line and take it the rest of the way.