Why did Karna receive gold instead of food in heaven?

According to the Mahabharata, Karna had never performed Pitru Tarpan or Pind Daan for his ancestors during his lifetime. In Svarga, the cosmic law of reciprocal duty (Pitru Rina) meant he could only receive what he had given — since he gave nothing to his ancestors, they could give him nothing in the form of nourishment. Lord Indra granted him fifteen days on earth to perform Pind Daan, and those fifteen days became the basis for Pitrupaksha.