Doctors give various reasons for not disclosing Alzheimer's disease as the actual diagnosis tot he concerned patient or their family.

Results showed that only 45% were told by the doctor about Alzheimer’s while 90% of people with the four most common cancers- breast, colorectal, lung and prostate said they had been told about their diagnosis.
Failing to promptly notify Alzheimer's patients of their diagnosis robs them of the chance to live life to the fullest and play an active role in planning for their future, since many learn of their illness only after their faculties have started to drastically decline, said Keith Fargo, Director of scientific programs and outreach for the association.
Doctors commonly cite fear of causing emotional distress as one of the main reasons they fail to disclose an Alzheimer's diagnosis. Uncertainty about their diagnosis, insufficient time to fully discuss treatment options and support services, a lack of support services, and the general stigma that surrounds Alzheimer's were some of the reasons for doctors not to disclose the diagnosis.
Source-Medindia