I have many cards of various breed’s of dogs but I have 4 full cards of just Scottie dog’s my theory why it is the most popular dog depicted on buttons is this
Fala (April 7, 1940 – April 5, 1952), a Scottish Terrier, was the dog of U.S. President Franklin D. Roosevelt.(he was pres. 1933 to 1945) One of the most famous presidential pets, Fala captured the attention of the public in the United States and followed Roosevelt everywhere, becoming part of his public image. Given to the Roosevelts by a cousin, Fala knew how to perform tricks; his White House antics were widely covered in the media and often referenced both by Roosevelt and his wife, Eleanor Roosevelt. Fala survived Roosevelt by seven years and was buried beside him. A statue of him beside Roosevelt is prominently featured in Washington, D.C.'s Franklin Delano Roosevelt Memorial, the only presidential pet so honored.