In his book 'Operation Overflight' Francis Powers explains that he chose not to use the ejection seat as he could not get himself in the correct position for ejection because of the g-forces he encountered. Had he used the seat, the canopy edge might have taken off his legs. Apparently the combination of the ejection seat type and the available space in the cockpit meant that a successful ejection could only be made from a very specific seating position. I assume that the more modern seat type in use now, coupled with the larger cockpit size of the newer U-2s, has at least removed part of this hazard.