The primary reason for focusing on art from the Old, Middle, and New Kingdoms of Egyptian history and not the Intermediate periods is:
• These were the "high points" of Egyptian civilization, when most of Egyptian artistic accomplishments occurred.
The Old, Middle, and New Kingdoms represent the most stable and prosperous phases of ancient Egyptian history, during which significant advancements in art, architecture, and culture took place. The Intermediate periods, on the other hand, were characterized by political instability and fragmentation, which often hindered artistic production and innovation.