The main reason is the lack of adequate preparation. It is simply not enough to acid wash concrete floors. the acid will only etch the soft parts of the concrete surface but will not etch the harden sections. Therefore this creates a poor bond between the surface and the paint, causing it to flake off.

Best practice is to grind the entire surface to ensure maximum adhesion.

Yes, even newly laid concrete releases laitance* that rise to the top surface. This creates a barrier between the coating causing a weak bond.

*Laitance is an accumulation of fine particles on the surface of fresh concrete due to an upward movement of water.