There's increased costs, for maintenance (regular cleaning) and replacement (it still cracks when damaged, even if it stays in one piece).
Glass by itself isn't nearly as strong as steel, so it would either need bollards or a steel fence to protect against vehicles. Vehicles crashing through gates can be very bad.
Bollards may not be a good idea though, because a smaller vehicle such as a motorcycle might still be able to go between the bollards and break through the glass.
Perhaps the lower half of the fence could be the current steel fence (to protect against large and small vehicles), and the upper half could be glass (to reduce the aesthetic impact).