Aesthetics and durability of ther finish. Repeated removal and installation of wheels leads to damage of the finish on and in the immediate area of the dropouts. The other common area for chroming, for durability, is the right chainstay, due to chain contact fromm wheel removal and chain slap. Paint is not as resistant as chrome, to this type of damage.
The drawback with chrome is the potential for hydrogen imbrittlement, its cost and it's toxicity. By the end of the steel era, it's use had been restricted primarily to dropout faces and drive side chainstays.
The irony of chrome is that while it is more resistant than paint when it comes to damage from impacts and abrasion, it is less resistant to rusting from moisture.