I think it’s because it is the very center of the country if you include Alaska and Hawaii. (Center of contiguous USA is somewhere in Kansas, I believe.)
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Actually:
“The geographic center of the US lies within private land near Belle Fourche, South Dakota. The geographical center of the US lies in South Dakota in the town of Belle Fourche. About 20 miles to the north of Belle Fourche at latitude 44 58 02.07622N and longitude 103 46.60283W lies the exact geographical center location of the 50 states of the US.”
http://www.penryfamily.com/geographicalcenters/lebanon.html
The determination of the Geographical Center at Lebanon, Kansas, came in 1918 when Charles H. Deetz of the U. S. Coast & Geodetic Survey calculated the new location which shifted the focus away from the previous location at Smith Center, Kansas. It wasn't until nearly 22 years later that an effort was made to mark the location and place a monument. On April 25, 1940, two surveyors from the firm of Paulette & Wilson in Salina, Kansas, determined the exact location by triangulation on the ground.
The location through survey measurements, placed the exact location in a cultivated field 600 yards north and 400 yards east of the nearest county roads. The owner of the land, John L. Grieb, did not want a monument placed in his field, so a decision was made to place it at the nearest intersection 750 yards to the southeast.
...snip...
When the Geographical Center location was made to include the states of Alaska and Hawaii in 1957-58, to be near Castle Rock and Belle Fourche South Dakota, the monument at Lebanon began to lose its significance. Promotional items were then required to add the words "Of the Continental United States" when referring to the location as the Geographical Center. Nevertheless, the location at Lebanon continues to draw many visitors each year. Over the years a chapel and picnic shelter have been added to the park.
Pictures--including a picture of Mr. Grieb with his horses and a sicklebar-- and maps at the link.