it's not that it's too heavy, its just that it's too expensive a process to do to an entire vehicle to make it effective. you'd have to strip down a vehicle to bare metal inside and out so you don't melt the interior/wiring/etc. then you have to worry about plastic bumpers trim, etc, can't powder those. and then you have to have an oven large enough to fit an entire vehicle inside it.it's also not as shiny as paint. it's much harder to get a deep gloss out of a powder than a traditional urethane paint. that said though, a lot of harley/cruiser guys are starting to explore powdercoat "paint jobs" on their bikes. powder has really come a long way in the past few years, and the colors, textures, candies, flakes, and metallics are of much higher quality and more varied.
the process ferrari, and many other makers, use to paint their vehicles is a powderized paint process, not actually a powdercoat. it uses low moisture fine particle paint because it reduces drying time, and paint usage, while still maintaining durability and paint coverage.