A. She has not done the type of legal work that would indicate she is qualified.
B. Many individuals with experience in the process of Constitutional law and the SCOTUS did not consider her to have the requisite qualifications for the SCOTUS.
C. She has not been identified as a person with such surpassing skills and a brilliant legal mind that lack of practicing qualifications as a judge would be secondary considerations.
To put it in sporting terms, the vast majority of major league baseball players spend time in the minors developing their skills before being promoted to the big leagues. A very select few superb talents can bypass the minors and go straight to the bigs. Miers hasn't played in the legal "minor leagues" that precede a SCOTUS justice-ship, and she hasn't demonstrated the skills and intellectual underpinnings one would expect in someone who bypasses the normal preparatory stages.