Because it means "Az az orvos fiatal". The first "az" means "that", the second "az" or "a" is required before the noun unlike in English.
"the doctor" = "az orvos", "that doctor" = "az az orvos", "this doctor" = "ez az orvos".
"Az orvos fiatal." - "The doctor is young" - This is a statement, a sentence with a subject, a verb and an adjective.
"A fiatal orvos" - "The young doctor" - This is just a description that can be an answer to a "who is" type of question.