Why gram positive cells are able to retain the crystal violet but gram negative retain safranin?

The answer is the thickness of the peptidoglycan layer.

Explanation:

Gram-positives have a simple cell wall with a thick layer of peptidoglycan which fixates the crystal-violet and KI complex while gram-negatives don't. Then when the destaining is done with alcohol, the gram-negatives are washes while the gram-positives are permanently stained. The last step is staining the gram-negatives with safranin which will happen but it will not be washed, and also it will not fixate in gram-positives.