You'll have to ask your doctor in order to know for sure what was meant in your case. Obesity and severe knee deformities are the most likely reasons why a patient wouldn't be considered for the new minimally invasive surgery (MIS).
Sometimes too much loss of knee motion is a problem. Other risk factors include brittle bones (osteoporosis), diabetes and/or smoking (poor wound healing), and previous surgery on that knee.
If a doctor is just learning to do the procedure, he or she might only take patients who have no known risk factors. Age may play a factor, as does the presence of other diseases or illness.