One issue with CBT is that it requires motivation. It can require up to 20 sessions and is a big comittment especially for those struggling to cope. Therefore many people may consider antidepressants and drugs an easier and quicker alternative as they require less effort from patients. furthermore
Patients with severe depression may not engage with CBT, or even attend the sessions and therefore using soley this treatment will be ineffective in treating these patients. Therefore for severely depressed patients other treatments such as drug therapy may be required before they are able to cope with the demands of the therapy.
cujipers et al found cbt works most effectively when used in conjunction with drug treatements