Taft’s positions and accomplishments, although they were Progressive by many standards - such as doubling the antitrust lawsuits of TR, constitutional amendments for federal income tax and direct election of senators - they were not always in line with the views of the Progressive wing of the Republican Party. In particular, Taft’s refusal to denounce the Payne-Aldrich Act infuriated many Progressives. Without going too deeply into the weeds, the Payne-Aldrich Act unexpectedly raised tariffs on many imports, thanks to successful politicking by Senator Nelson Aldrich. As a general rule, Republicans were strong proponents of protectionism, as higher tariffs meant higher profits for American businesses. Progressives, however, believed that tariffs promoted the growth of monopolies and trusts. Taft had campaigned on tariff reform, and those who expected lower rates were shocked and angered when the bill that was passed instead continued the high rates that had begun under the McKinley administration. The split between the Progressives and the Old Guard would only grow wider from here, costing them both the 1910 midterms and the 1912 presidential election. For his part, Taft actually thought he would be preserving the unity of the party in signing the bill:

Now, I am not here to criticise those Republican Members and Senators whose views on the subject of the tariff were so strong and intense that they believed it their duty to vote against their party on the tariff bill. It is a question for each man to settle for himself. The question is whether he shall help maintain the party solidarity for accomplishing its chief purposes, or whether the departure from principle in the bill as he regards it is so extreme that he must in conscience abandon the party. All I have to say is … that I believed that the interests of the country, the interests of the party, required me to sacrifice the accomplishment of certain things in the revision of the tariff which I had hoped for, in order to maintain party solidarity, which I believe to be much more important than the reduction of rates in one or two schedules of the tariff. (William Howard Taft, Address on the Tariff Law of 1909. https://www.presidency.ucsb.edu/documents/address-the-tariff-law-1909)

This was the first of two major blows to the relationship between Taft and the Progressives. The second was a dispute known as the Ballinger-Pinchot Affair (or some variation of that name). In the beginning of his administration, Taft fired Secretary of the Interior James Rudolph Garfield and replaced him with Richard A. Ballinger, former Commissioner of the General Land Office. Ballinger generally favored private development of land over public conservation, and would soon be accused of illegal and improper dealings regarding coal claims in Alaska by the Morgan-Guggenheim syndicate. Chief Forrester Gifford Pinchot, who had been appointed by Roosevelt and was a champion for conservation, began to openly criticize Ballinger, and soon Taft himself. Taft responded by firing Pinchot for insubordination. Ballinger's investigation was a sham. Conservationists were livid - especially Theodore Roosevelt, who was Pinchot's close friend - and Taft's relationship with the Progressives had been damaged irreparably.

(For the original article that caused a national scandal, see Collier's magazine, November 13, 1909, "Whitewashing of Ballinger," https://archive.org/details/colliers4419unse. For an example of how fearful people were that the trusts could take over public lands, see this cartoon, "The United States of Guggenheimerica," https://digital.library.cornell.edu/catalog/ss:3293854)

These two events demonstrate an important point about William Howard Taft as president. It is less that Taft was seen as totally business-friendly - his antitrust lawsuits certainly disproved that - and more that he was seen as someone who could not deliver on Progressive promises. And this perception of Taft as not a true Progressive also has to do with the influence of Roosevelt himself. When Roosevelt returned to the United States in the summer of 1910 from his African safari and European tour, he began speaking loudly and openly against the Taft administration. Although he was handpicked by Roosevelt, and on certain issues they shared views, they were not identical. Furthermore, due to his leadership style, Taft was seen as weak and ineffectual compared to TR. Whereas Roosevelt essentially invented the modern presidency, pushing the boundaries of presidential power in the vein of Jackson and Lincoln before him, Taft felt less comfortable with the “bully pulpit” and exercising too much power over legislative processes. As the Miller Center at the University of Virginia notes, “His conception of executive leadership was primarily focused on administration rather than legislative agenda-setting.” (Miller Center, William Taft: Domestic Affairs, https://millercenter.org/president/taft/domestic-affairs)

Roosevelt made speech after speech claiming that Taft did not deserve to call himself a Progressive. (Theodore Roosevelt, "What a Progressive Is," 1912. https://www.theodorerooseveltcenter.org/Research/Digital-Library/Record?libID=o285264) He even accused Taft and the Republicans of underhanded methods such as stuffing the ballot box. (https://www.theodorerooseveltcenter.org/Learn-About-TR/TR-Quotes/The%20control%20of%20the%20Republican%20National%20Convention%20in%20June%20%201912%20%20in%20the%20interest) His criticisms were effective, and many voters agreed with his assessments. For example, in January 1912, a New Mexico rancher wrote to TR to ask him to “come out in the open” with his campaign, and that he could not support Taft, who was “lacking in back bone.” (Letter from Harry R. Hannum to Theodore Roosevelt. https://www.theodorerooseveltcenter.org/Research/Digital-Library/Record.aspx?libID=o74779) Taft ultimately came in a distant third to Wilson and Roosevelt, in no small part thanks to the perception that he was not the Progressive he claimed to be. His competitors had strong platforms and strong personalities. Roosevelt had his “New Nationalism,” and Wilson his “New Freedom,” and Taft had to get a new job.