An expanded/more comprehensive revisit of a topic from years past...Over the long history of the classic era of US commemorative coins, there were several instances in which coins were sought for similar/related anniversary milestones, with only one making it to the finish line and into struck coinage. Here's a quick recap:
Oregon Trail MemorialThe Oregon Trail Memorial Half Dollar did not formally mark a specific date in US history, but rather a period of time during which a great western migration took place. Among the coin program's objectives was the raising of funds to pay for markers to be erected along the original trail so that its memory would not be lost.
The coin program featured multiple issues over its 1926 to 1939 run, but did not exhaust its authorized maximum mintage. In 1947, the Idaho Unit of the Oregon Trail Monument Association sponsored a bill in Congress that was to make use of the unstruck coins from the original Oregon Trail coin program. Fortunately, the 1947 bill did not succeed - most collectors were not clamoring for more "Oregon"half dollars!
1947 Oregon Trail Memorial - Revival-
1947 Oregon Trail Memorial Revival (Idaho Unit)1926 Oregron Trail Memorial Half Dollar
Vermont SesquicentennialThe "Bennington" half dollar was a dual-objective commemorative coin that was designed to mark the sesquicentennial (150th anniversary) of the Battle of Bennington and the independence of the Republic of Vermont from Great Britain (Vermont was an independent republic before it became a State in 1791.)
In 1941, a commemorative half dollar was sought to mark the 150th anniversary of Vermont Statehood; the bill did not succeed.
1941 Vermont Statehood Sesqui-Centennial-
1941 Vermont Statehood Sesqui-Centennial1927 Battle of Bennington- Vermont Independence Sesqui-Centennial Half Dollar
Wisconsin CentennialCollectors are often familiar with the 1936 Wisconsin Territorial Centennial Half Dollar, but if Congress had been more receptive, they might also be familiar with a coin that marked the 100th anniversary of Wisconsin's Statehood Centennial. Such a coin was proposed for 1948, the State's Centennial year, but it was not approved - the Wisconsin Centennial had to make do without a commemorative half dollar!
1948 Wisconsin Territorial Centennial-
1948 Wisconsin Statehood CentennialWisconsin Territorial Centennial Half Dollar
Iowa CentennialThough well-known today, the Iowa Statehood Centennial Half Dollar was not the first coin proposed to honor a major milestone in Iowa's governmental history. As Iowa was a US Territory before it was a US State - and Wisconsin's Territorial Centennial had just been marked with a commemorative half dollar - a proposal was made in Congress to strike a 1938 Iowa Territorial Centennial Half Dollar. The proposal did not succeed, but Iowa's Statehood Centennial was deemed worthy of a coin and thus we have the 1946 half dollar with which US commemorative coin collectors are familiar.
1938 Iowa Territorial Centennial-
1938 Iowa Territory Centennial1946 Iowa Statehood Centennial Half Dollar

There was also a "double-dip" attempt that failed on both ends. You can read about the related pair via the following links:
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1949 Minnesota Territorial Centennial-
1958 Minnesota Statehood CentennialFor more of my stories about commemorative coins and medals, including more on the successful half dollars imaged above, see:
Commems Collection.