Why do we use Ziehl-Neelsen stain, not gram stain for mycobacterium tuberculosis?

Ziehl-Neelsen stain, also known as the acid-fast stain, is a special bacteriological stain used to identify acid-fast organisms, mainly Mycobacteria. Acid fast bacteria are stained bright red and other organism and background will be blue. Acid-fast bacteria are bacteria that resistance to decolorization by acids during staining procedures.

Why do we use Ziehl-Neelsen stain, not gram stain?
Acid fast bacteria cell walls contain peptidoglycan but they have a waxy coat on their cell wall too. Gram stain (crystal violet nor the counterstain (safranin)) can not penetrate the waxy layer. Therefore they will not be visible able to see under the microscrope.