I don’t think it’s exactly a matter of appealing to progressives, but an inherent difficulty in trying to appeal to a wide variety of people with a wide variety of interests. You end up having to make vague promises in order not to offend anyone, which comes across as being boring or disingenuous, like you are only saying stuff in order to get elected and not because you actually belive in it. Conservatives can have much simpler and more straightforward messaging because their base is much more homogeneous.
Wait, do you think that the united states is less progressive than it was 10 years ago? How about 20 years ago? 30?