How about because there was no such plot in the first place?
Consider the proposal for a vast march of veterans on DC as a key point in the conspiracy, the plotters somehow oblivious to such an event having actually taken place when the Bonus Expeditionary Force came to town.
Or the same plotters being unaware of Butler's publicly known positions, and then thinking MacArthur would be a good second choice, under the delusion that MacArthur enjoyed great popularity with American veterans when, after his handling of the BEF, the average veteran might sooner shoot MacArthur than listen to the man.
Consider the proposal for a vast march of veterans on DC as a key point in the conspiracy, the plotters somehow oblivious to such an event having actually taken place when the Bonus Expeditionary Force came to town.
Or the same plotters being unaware of Butler's publicly known positions, and then thinking MacArthur would be a good second choice, under the delusion that MacArthur enjoyed great popularity with American veterans when, after his handling of the BEF, the average veteran might sooner shoot MacArthur than listen to the man.