Based on what I found on the internet, she withdrew from Nationals in 1996, which was the year after she won bronze at Worlds and had been 1995 National champ.
She did not have the best reputation about about practicing and conditioning. It surprised me how out of shape she got between 1994 Nationals and 1994 Worlds. She stopped training when she didn't make the Olympic team and it showed visibly when she didn't pass the QR at World's and while she was on COI.
I'm guessing that Nicole might not have passed her 'test' or was not able to get a doctor's note citing the liklihood of recovery. I'm assuming that she should have healed instead of touring and that the overall injury may have required more recovery time. Michelle has already been recuperating for several weeks from the groin injury.
If anything, one should compare Nicole's request with Nancy's. Both were injured at Nationals with no significant recovery time before the World team was announced.
Nancy had a horrific 1993 season, which cost the US an additional World team spot. This has been perceived as her personal mental blocks and the added media attention just from her being the favorite in 1993. After the 1993 season, Nancy went to a sports psychologist and practiced full run throughs of her programs (note; during interviews, she said that she had not been doing full run throughs because of a fear of not being able to and was always more interested in practicing her exhibition routines) and arrived at Nationals in the best shape of her life - both physically and mentally. At Nationals, she is attacked and cannot compete. Based upon her skating test, she is put on the team.
Nicole had a great 1995 Season - 1st at Nationals and 3rd at World's. Her placement and Michelle's guaranteed 3 spots at the 1996 World's. She does Nutcracker on Ice, where she becomes injured. Instead of rehab'ing the injury, she continues to tour. She shows up at Nationals injured and in less than peak conditions aerobically. Nicole aggravates her injury further and has to withdraw from Nationals.
As you can see, there is a bigger argument for putting Nicole on the World team than there was for putting Nancy on the Oly team. So again, my best guess is that Nicole could not find a doctor who would certify that she could possibly recover in time and/or could not pass her skating test.