Final answer:
William Howard Taft disappointed the Progressives due to his conservative leanings on tariffs and his failure to leverage the presidential influence for progressive reforms. Signing the Payne-Aldrich Tariff and supporting conservative candidates were key issues that led to the disappointment.
Step-by-step explanation:
In the end, William Howard Taft disappointed the progressives in his party because he was not progressive enough. Over the course of his presidency, Taft was hesitant to fully support the progressive agenda, instead often siding with the conservative wing of the Republican Party. He alienated Progressives with his handling of tariffs, particularly with the signing of the Payne-Aldrich Tariff, which thwarted their effort to lower tariffs, and instead raised them. Additionally, the rift between Taft and the Progressives was widened with his support for conservative candidates in the 1910 elections and the creation of the U.S. Chamber of Commerce, which was seen as an attempt to counter the labor movement.