I had to meditate on how to answer your question for a moment, as it is slightly involved.

There are actually a few different answers to be found, but "economic considerations" as we would see them today, is the most unlikely. Let's start with the change in society. New Kingdom Egypt is when we can start to see the full power of the Egyptian state. With that power comes a slightly more pronounced form of power projection and administration. "In the Old and Middle Kingdoms, monumental architecture was kept at the periphery of the visible world" states Barry Kemp's "Ancient Egypt: Anatomy of a Civilization". That meant Pyramids and large scale construction were usually meant for the public to see from afar, and typically smaller scale temples and buildings were placed in the residential towns of the people. We're also talking about mud-brick materials being used instead of stone. However, the New Kingdom moved that monumental architecture and those stone materials INTO the cities of the people. We see this reach it's height with buildings such as the Temple of Karnak in Thebes(Waset) which can attribute most of its' construction to the New Kingdom. But they also continued with building on the outskirts of their civilzation as exemplified by the Temple of Hatshepsut

Religiously, the nature of tombs also changed with the New Kingdom. Previously, the Pyramids seem more dedicated to the Sun-Cult. Now, the mortuary temples all point to Amun specifically. And within that subject, the various temples pointed to a specific form of Amun. So that different leaders devoted their temples to different forms of Amun.

Another reason behind this change was security. A gigantic Pyramid is an obvious target.... Now, they would use a catacomb system that left some treasures untouched by tomb robbers till our day! Also, the people involved in the tomb building in the Valley were now a special workforce that would live out their days in the desert.

So it was not as if they lost their architectural ability. And economically, they were actually in a better position than the Old and Middle Kingdoms, as more loot and tribute was flowing into the "Empire" than ever before. But societal and administrative changes seemed to push a change in the way that mortuary temples were developed.

Some good sources for this information are Barry Kemp's outstanding book mentioned above, "Thebes in Egypt" by Nigel and Helen Strudwick, and "Village Life in Ancient Egypt" by A.G. McDowell which gives details on the life of the workforce in the valley.

Hopefully this helps you out.