Why aren't there many diesel supercars and hypercars?

Because diesel isn't traditionally designed for that sort of use, where you need sudden explosive power. Diesel's main benefit is in steadily chugging along at constant speed over long distances, which is where its fuel economy etc. advantages become apparent. Also, for a given power, a diesel engine is always going to be slightly heavier, thus giving a worse power-to-weight ratio. It's not to say that you can't have a high-performance diesel engine, modern diesels are certainly gradually narrowing the performance gap, but nevertheless that gap does still remain.