Here is my opinion of the reason: Conservatives have a huge outside-the-party infrastructure devoted to persuading the public to support their policies and progressives do not.
Conservatives recognize the value of movement-building and work steadily to create popular demand, which then gets their candidates elected. This is why so many terrible Republicans are able to get elected just by pointing their finger at their opponent and shouting, "Liberal, Liberal!"
Progressives instead for decades have believed that a candidate will come along who will be so popular that he or she will lead them out of the wilderness, and convince the public of the rightness of all of their ideas. Therefore almost all of their money and effort goes into short-term election efforts, candidates and the party instead of to ongoing outside-the-party organizations that work over the long term to build lasting demand for their ideas.