Early designers of steel plate shear walls (SPSWs) often overlooked the system's post-buckling behavior due to several factors:

1. Lack of Understanding: In the early stages of SPSW development, there was limited understanding of the complex behavior of steel plates under lateral loads. Designers primarily focused on the initial elastic behavior and the overall strength of the system, neglecting the nuances of post-buckling performance.

2. Simplified Design Models: The design methodologies available at the time were often based on simplified models that did not adequately account for the non-linear behavior of steel plates after buckling. Many design codes and guidelines were not updated to reflect the advancements in understanding of material behavior under extreme loading conditions.

3. Emphasis on Strength Over Ductility: Early design practices tended to prioritize strength and stability over ductility and energy dissipation. This led to designs that did not consider how the shear walls would behave after reaching their buckling capacity, which is critical for ensuring the structure's resilience during seismic events.

4. Limited Experimental Data: There was a scarcity of experimental data on the post-buckling behavior of steel plates, which made it difficult for designers to accurately predict how these systems would perform under real-world conditions. As a result, many design decisions were made based on theoretical models that did not fully capture the complexities of post-buckling behavior.

5. Evolution of Design Philosophy: The design philosophy for structural systems has evolved significantly over the years. Early designers did not have the benefit of modern computational tools and advanced analysis techniques that allow for a more comprehensive understanding of post-buckling behavior.

As research and testing progressed, the importance of considering post-buckling behavior became more recognized, leading to improved design practices and codes that better account for the performance of SPSWs under lateral loads. This evolution has helped enhance the safety and reliability of structures utilizing steel plate shear walls.