Considering the dominant position held by the Half Dollar in terms of "Denomination Frequency" within the classic US commemorative coin series, it's easy to overlook the fact that the Gold $1.00 coin was used most frequently for US commemorative coins until 1920.
A look at the statistics prior to 1920:
Denomination Frequency Coin Notes
Quarter Dollar 1 1893 Isabella
Half Dollar 3 1892-93 Columbus, 1915 Panama-Pacific Expo,
1918 Illinois Statehood
Dollar, Silver 1 1900 Lafayette
Dollar, Gold 5 1903 LPE-Jefferson, 1903 LPE-McKinley,
1904-05 Lewis & Clark, 1915 Pan-Pac,
1916-17 William McKinley
Quarter Eagle 1 1915 Pan-Pac
Quintuple Eagle 1 1915 Pan-Pac
In 1920, the Half Dollar pulled even with the Gold Dollar (at five design types) with the release of the 1920 Landing of the Pilgrims Tercentenary and 1920 Maine Statehood Centennial coins. The tide turned forever in the favor of the half dollar in 1921 when the 1921 Alabama Statehood Centennial and 1921 Missouri Statehood Centennial Half Dollars were issued. This pair gave the half dollar a lead that it never relinquished in the classic series - it only stretched it!
A look at the statistics after the series completed in 1954:
Denomination Frequency
Quarter Dollar 1 (2 if you include the 1932 Washington Quarter)
Half Dollar 48
Dollar, Silver 1 (2 if you include the 1921 Peace Dollar)
Dollar, Gold 6
Quarter Eagle 2
Quintuple Eagle 1
Why the early use of the Gold Dollar?
One possibility is the fact that the Gold Dollar had not been struck since 1889, and was not a dominant coin in circulation. Its use for commemorative pieces would have had minimal impact on the US coinage system. It was also believed by sponsors that the public was nostalgic for the gold dollar after being absent for more than a decade.
For other of my posts about commemorative coins and medals, including more on each of the commemorative coins referenced here and other "Quick Bits" stories, see:
Commems Collection.