The reason is to give the nightfall ordeal playlist a reason to be played. Bungie is scrapping tooth and nail to try to extend playtime, and while raids CAN potentially take a long time, it’s not always the case if you get the right type of people. Bungie’s stinginess with upgrade materials is purely to extend grind. While raids are typically difficult, bungie wants only the hardest content in the game to drop it (and this is also the reason the drops are NOT guaranteed on a nightfall completion. Extend your grind further and to force you to play continuously. Bungie more then likely dreads the fact that they were forced to add double nightfall weeks as this alone greatly cuts the grind, but the results were overwhelmingly clear when the original glitch came out that it alone attracts people to play it more).