Bling, pure and simple, and 'marketing'. Some people think they look better and are prepared to suffer the discomfort, vulnerability to damage and high replacement costs of the tyres. As long as smaller wheels with deeper profile tyres will fit around the front brakes, they will offer far better ride comfort, be less vulnerable to damage, will last longer and will; be cheaper to replace. The 'marketing' element is because tyre makers gain huge profits from replacement low profile tyres, they discount them to car manufacturers.